Question

A merchant sells three different sizes of canned tomatoes. A large can costs as much as 5 medium cans, or 7 small cans. If a customer buys an equl number of small and large cans of tomatoes for the exact amount of money that would buy 200 medium cans, how many small cans will she buy?

Answer 1

she can buy 280 small cans.

Answer 2

let L = cost of 1 large,
M = cost 1 medium and
S = cost of 1 small can
now we can write some equations relating these variables:
L = 5M
L = 7S
200M = xL + xS     where x = number of small and large cans
Since 5M=7S
S=5M/7
finally 
     200M=x(5M)+x(5M/7) from this you'll get  x=35