3 oranges= $1.35, so one orange cost $0.45 4 oranges= $1.80, one orange cost $0.45 5 oranges= $2.25, one orange cost $0.45 If one orange cost $0.45, then ten cost $4.50 ($0.45*10)
If you were to buy ten oranges are $4.25, this would make each orange $0.425 ($0.43 rounded).
This means that if you were to buy ten oranges, it would be cheaper to buy ten for $4.25, instead of buying two lots of 5 for $2.25 each.