Try to start from the end. So draw out 1 unit for Susan and 4 units for Iris. Then, we cut away 50 from iris and give back 50 to Susan. We also know Susan had 10 more initially than Iris. So we can draw the dotted lines then Susan had 10 more. So we take the whole purple part from Susan and minus away 10 to get the red part. The red part plus 50 from iris gives you 3 units. Solve what is 1 unit then we find out susan’s Original amount, which is 1 unit plus 50. Hope it helps!