First, find a common denominator. The closest would be 10. Multiply the fractions so the denominator becomes ten. That would mean 2 1/2 become 2 5/10 and 3 2/5 would become 3 4/10. Then add them together it would become 5 9/10. Finally you can subtract 5 9/10 from 8 and you would get 3 1/10 left.