The part you cannot see

Draw it before you answer it. A tank holds 18 litres. There are 11 litres in it.

Do not answer yet. Draw the bar, and put a box where the missing number goes.

Same picture as the last two lessons. The only change is that one of the two parts is not written down.

$6 + \square = 9$ and $9 - 6 = \square$ are not two questions. They are one picture, read down and read up.

So "how many more" and "how many are missing" are the same sum, and you never have to decide which one a question wants. Draw the bar. The bar decides.

Add or subtract? It turns on one thing: whether the whole is what you do not know. Missing whole, add. Missing part, subtract. Nothing else in the wording matters.

"Altogether" means add, "left" means subtract. It usually does, which is exactly what makes it dangerous: it works often enough to become a habit, then fails on the ones that count.

Amina sold 7 mangoes and had 5 left. The word is "left". The answer is $7 + 5 = 12$, because what is missing here is the whole.

It tells you which operation. It does not make you quick. You can read $8 + \square = 15$ off the picture and still pause over the answer — and a pause every time is what stops you thinking about the problem.

So: what would have to be true for that pause to disappear?