13. Mrs. Perry has a sum of money to buy fruits. She can buy n mangoes at $1.60 each and have $0.80 left. Alternatively, she can buy (n + 10) apples at $0.70 each and have $0.10 left. 
 (a) Find the value of n. 
(b) How much money does Mrs. Perry have for buying fruits?
(c) If Mrs. Perry buys 3 mangoes and uses the rest of the money to buy apples,
 (i) how many apples can she buy? 
(ii) how much money will she have left?