Thanks. Often confuse when to use multiple or minus in such question. Any advise?

The “kg” is often the difference between multiplication and subtraction.

Mrs Lim had 4/5 kg of flour. She used 1/5 of it…

Versus

Mrs Lim had 4/5 kg of flour. She used 1/5 kg of it…

One of this involves multiplication, while the other does not. Do you know which one?

Hi, 1) multiplication 2) subtraction.
Is my understanding correct? Thanks.

Yes - and it’s all because of the “kg”. Because 1/5 of it means cutting the “4/5 kg” (which is basically 800 grams) into five equal parts before we take one of the parts. Each of these parts would be 4/25 kg (which is basically 160 grams). To find the remaining quantity (if required), we then take 4/5 minus 4/25, since these are kg values.

1/5 kg of it, on the other hand, means that we literally remove 1/5 kg from the available quantity. This is already a kg quantity, so to find the remaining quantity (if required) we can subtract the value directly - that is, 4/5 minus 1/5.