salary ——- 40 units
(1/4) x 40 units = 10 units (bag)
40 units – 10 units = 30 units
(1/6) x 30 units = 5 units (shoes)
225 (scarf)
2/5 of the rest of her money (mother)
180 (son)
(3/10) x 40 units = 12 units (left)
3/5 of the rest of her money ——- 180 + 12 units
1/5 of the rest of her money ——- 60 + 4 units
5/5 of the rest of her money ——- 300 + 20 units
300 + 20 units + 225 ——- 40 units – 10 units – 5 units = 25 units
1 unit ——- (300 + 225)/5 = 105
40 units ——- 105 x 40 = 4200

Ans : $4200.

2016 Nanyang P6 SA1 Q18

AF, 40u (branch)

remaining 1/4 -> 10u

left -> 30u

spent 1/6 + 225

remaining- > 25u – 225

spent 2/5 

remaining-> 15u -135

ITE, 3/10 of her salary left ->3/10 x 40u = 12u

15u – 135 = 12u +180

15u – 12u = 180 +135

3u = 315

1u =105

40u = 4200 (ans)

Check here 

https://www.kiasuparents.com/kiasu/question/30674-2/

tq!