Jack's mother spent $18 on some English assessment books. She spent 7/12 of her remaining money on some Chinese storybooks. Given that she was left with 1/3 of her original sum of money, how much money did Jack's mother spend on the Chinese storybooks?
Let the amount of money she started with==M
M - 18 - what she had left after buying some English books.
5/12[M - 18] - what she had left after buying some Chinese books
5/12[M - 18] ==1/3M, solve for M
M ==$90 - amount of money she started with
7/12[90 - 18] ==$42 - she spent buying Chinese books.