Do that way if is easier

John's salary is R$900.00. It is known that 2/5 of his salary is spent on paying bills, 1/10 on entertainment and 3/1 on food and clothing. Apart from these expenses, the amount left over from his salary is:

A) R$ 150,00;

B) R$ 200,00;

C) R$ 450,00;

D) R$ 700,00;

E) R$ 750,00.

So to make that very fast: multiply everybody but the last by 100%

so 2/5 x 100% = 40%

1/10 x 100% = 10%; sum is 50% half of it 450,00 no paper spent for that and 1/3 is 300;

so 450 more 300 is 750 who need paper for subtract that from 900 is 200 less 50 that mkes 150.

Rute Bezerra de Menezes Gondim