The answer works itself out

Challenge 2 of 16

Notice what you never type here: the answer. You give the two numbers, the code does the adding, and the total appears on its own — which also means one wrong number makes the total wrong with it.

Set the missing number to match the target.

Your code JavaScript

Set the missing number to match the target.

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Lesson Adding Numbers

Adding is written the way you would write it on paper: var x = 2 + 3; The sum is worked out first, and its answer — 5 — is what ends up in the box.

That is worth pausing on, because it means you never type the answer. You give the parts and the sketch does the arithmetic. Change one of the parts and the answer changes with it, without you touching anything else.

The parts do not have to be numbers you typed. They can be other boxes: var total = a + b; makes total hold whatever a and b add up to at the time. Set a to 3 and b to 5 and total is 8; change a to 10 and total is 15, on its own.

A sum can be as long as you like — var total = maths + science + art; adds all three.

Adding is forgiving about order: 3 + 5 and 5 + 3 both make 8, so a pair of numbers can go in either way round. That stops being true in the next topic.

In this topic you will fill in missing numbers to hit a total, keep several sums at once, and then add boxes together by name.