This is an External Unequal Change question, where both Mrs Tan and Mrs Ismail has additional (and unequal) money coming from external. This is a very common kind of question for P6. And there are 3 types of such question. (Both add, both minus, or 1 add 1 minus)

The percentages given are for us to derive the Before and After Ratios. And because both ratios are not the same, thus we let Before ratio be in Units, and the After ratio be in Parts. When solving, make parts the same for both Tan and Ismail. From there we can draw a model and see that 5u = $400.

Once your child is familiar with such question, can skip model drawing and it will save some time. :)