Drawing a model helps to solve this question.
Wei Ling saved 1/2 of her allowance and so her allowance is split into 2 equal parts. Of the remaining 1/2, 2 out of 5 was spent. Hence we divide the remaining into 5 equal parts and shade 2 parts to represent the units spent. 3 units are left and this is equals to $51. After finding that 1 unit is $17, we proceed to find the allowance at first which should be twice the remainder (5u). So we find 5u = 5 x 17 and then we double it to find the allowance.
Hope this helps!