one Brian's item cost: x
one Chris' item cost: x+2.5
Set up equation:
Brians amount = Chris amount
4*x = 3*(x+2.5)
solve for x:
4x = 3x + 7.5
x = 7.5
Brian's items cost $7.5 each.
Check:
Brian paid 4*7.5 = $30
Chris paid 3*10 = $30
The individual cost of Brian's items was $7.50, and the indiv. cost of Chris' items was $10.