Name what's left

Challenge 4 of 16

A box holding a take-away can be named for what it means, and left says it plainly: what remains once something has gone.

Take the right number from 12 so left matches the target.

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Take the right number from 12 so left matches the target.

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Lesson Taking Away

Taking away is written with a minus sign: var x = 10 - 4; The sum is worked out first and the answer, 6, is what goes into the box — the same way adding worked.

One thing is different, and it is the thing to remember. Adding did not care which number came first: 3 + 5 and 5 + 3 both make 8. Taking away cares. 11 - 3 is 8, but 3 - 11 is -8, the same number with a minus sign in front. So the amount you start from goes first, and the amount coming off it goes second.

A negative answer is not an error. It is a real number, and sometimes it is the right one — a football team that lets in more goals than it scores has a negative goal difference. But if you did not mean to get one, it usually means the two numbers are the wrong way round.

Amounts can come off one after another, left to right: var left = start - monday - tuesday; takes Monday off the start, then Tuesday off what remains.

And as with adding, the numbers can be boxes: var difference = big - small; never needs setting by hand. Change either box and the difference follows.

In this topic you will fill in missing numbers, keep several take-aways at once, and then subtract boxes by name.