Copyright © 2015 jsd

Algebra at Khan Academy
John Denker

1  Overview

Khan Academy offers a thing called “Mission Algebra” which claims to give you “a personal path through Algebra I”.

The site will ask you some questions. If you are already mostly proficient, it goes pretty quickly. If you need to brush up on stuff, it will suggest exercises for you to do, and suggest training videos for you to watch.

2  Getting Started

If you want to go down this road, then:

3  Using the Site

3.1  User Interface

You will have to make allowances for the amateurish and inflexible user interface. In particular, suppose you have correctly answered a question and want to go on to the next. You would expect to find a “Next” button. Sometimes there is a button labeled “Correct! Next question ...”, but sometimes there isn’t, in which case you have to find the button that says “Awesome! Show points”. Whether or not you care about points, this is the only way to move on to the next screen.

Note that a typical user interface has a fixed set of buttons with fixed meanings in fixed positions, each of which may be active or inactive. Instead, KA has relatively few buttons, and changes their labels and meanings. So even though you know what a button did a moment ago, it may not do the same thing now. You have to read the fine print. This is nonstandard and unhelpful.

Beware that clicking anywhere on the margin of the page will take you away to a “Misson Progress” summary page. Very obnoxious. The designers evidently don’t realize that some computers nowadays (i.e. for the last 30 years) sometimes have more than one window open, and it is necessary to click somewhere to refocus and raise the window.

Just to rub salt into the wound, you cannot use the browswer’s “Back” button to return to where you were.

If you remember what task you were working on, there is probably a button on the summary page that will take you there.

On the other edge of the same sword, this is wrong in another way: If you wanted to go to the summary page, it’s not at all obvious how to do it. Clicking on the margin of the page is undocumented and highly non-obvious way of doing it.

Also beware that on multiple-guess questions, clicking on the page anywhere on the same row as a possible answer will change your answer. This is highly nonstandard user-interface behavior, and can lead to questions getting scored wrong, even if you knew the answer and entered it correctly.

3.2  Hints

Each ’‘practice” quiz page offers hints. Beware that looking at the hint guarantees that the question will be scored as a wrong answer, even if after seeing the hint you enter the correct answer. This can be very confusing, because the “wrong” answer will become part of your score, even though you never typed in a wrong answer.

In addition to being confusing, it’s unfair, in situations where you just wanted to see the hint out of intellectual curiosity ... or in cases where the hint was unhelpful.

Constructive suggestion: In at least some cases, you have the option of looking at the hints after you have entered and checked the correct answer. This incurs no penalty. Beware that if you go to the summary page and back to the task (as discussed in the previous item) you lose this option.

4  Miscellaneous Observations

There are multiple questions of the following general form:

In the formula d=m/d, the variable d is density, m is mass in grams, and v is volume in cubic centimeters. What are the units of density?

That’s the wrong way to think about units and dimensions. I say m should represent the mass, no matter what units are being used. You should not write mass = m grams, but rather mass = m = some number of grams. Similarly for the other variables.

I say there is no such thing as “the” units for density. There are dozens of common units for density, and probably hundreds of other usable units.

This looks to me like rote, mechanical manipulation of symbols without regard to what they should mean.

Problem Elsa and Jolie earned money for babysitting. In one month of
babysitting, Elsa earned $205.50, and Jolie earned $x. They earned a
total of $400 all together.  Write an equation to describe this
situation. Do not solve the equation for x.

Jolie’s earnings should be written as x (not $x).

Similarly, in the section on units, the question is

How much will it cost Jennifer to fulfill the cupcake order?

Alas $40.00 is not an allowed answer. You have to enter 40.00 without the dollar sign. You have to throw away the units to get credit. This teaches seriously bad ideas about units.

Saki is transporting barrels of oil from Alaska to Florida. He is using a semi-trailer truck that has a weight capacity of 20,000 kg. Each barrel of oil weighs exactly 57 kg.

That’s bogus data. The “barrel” of commerce is 42 gallons. The drums shown in the picture hold 55 gallons. Oil weighs on the order of 6 pounds per gallon, so each barrel weighs well over 100 kg.

Interesting type of question:

A parking garage has nine levels and is 30%
full. Each level is separated into a different
number of sections. What quantity would be most
useful in determining how many more cars can park in the garage?

    Total number of parking spaces per section
    Total parking spaces in the garage
    Total number of sections per level


Giovanna spent $12\$12$12 on kilograms of blueberry scones.
Which quantities would be useful for finding the number of scones she bought?

    Select all that apply.
    Price of one scone
    Price per kilogram of scones
    Average mass of one scone
    Average number of blueberries per scone

What on earth is this??????????? Countdown timer:

Mastery challenge available in 09:47:08

Why is this on a timer? Why is it not performance-based?????

Time now: Wed 22 Apr 2015 11:21:07 PM MST

Possibly related:

Finish just one more mastery challenge to get the Geek of the week: mastery badge

And then I do a "mastery challenge" ... but I don’t get the badge. It still says I need to do one more. Wed 22 Apr 2015 11:31:40 PM MST

Copyright © 2015 jsd