When each girl spent the same amount of money, the difference between the girls’ money remains the same.

At first,
J:S = 2:3

Difference between Janice and Siti is 1 unit. (3-2=1)

In the end,
J:S = 3:5

The difference now becomes 2 units. (5-3=2)

To make the difference the same, we need to multiply the ratio at first by 2.
J:S = 2:3 = 4:6

By doing so, the difference between Janice and Siti will become the same (2 units).

Now, we just need to compare Janice’s (or Siti’s) money at first and in the end.
J → 4-3 = 1
S → 6-5 = 1

By comparing at first and in the end, it’s clear that:
1 unit → $32 (money spent on school bag)

Siti’s money (in the end):
5 units → $32 x 5 = $160