Lets x = the cost of a carton of eggs and y = the cost of a bottle of milk
Monday, he sold 5 cartons of eggs and 10 bottles of milk for $33.50
5x + 10y = 33.50
Tuesday, he sold 4 cartons of eggs and 7 bottles of milk for $24.30
4x + 7y = 24.30
Now you have 2 equations
5x + 10y = 33.50 (1st)
4x + 7y = 24.30 (2nd)
Multiply (1st) equation by 7 and (2nd) equation by 10
35x + 70y = 234.5 (1st)
40x + 70y = 243 (2nd)
----------------------------------subtract
-5x = -8.5
x = 1.7
Answer
The cost of a carton of eggs is $1.70
Hope it helps
