7 apples and 5 bananas cost $6.30. 11 apples and 7 bananas cost $9.60. Based on this information, how much will 15apples and 13 bananas cost?
7a + 5b = 630 --- times 7 ---> 49a + 35b = 4410
11a + 7b = 960 -- times 5 ---> 55a + 35b = 4800
subtract them
6a = 390
a = 65
sub into 7a + 5b = 630
455 + 5b = 630
b = 35
now you know the value of a and be, so find 15a + 13b