Let the number of bags to be purchased be x
Option Val : 250 + 1.50x
Option Sally : 100 + 4x
For Val to be the cheaper option:
250 + 1.5x < 100 + 4x
100 + 4x > 250 + 1.5x
100 + 4x - 100 > 250 + 1.5x - 100
4x > 150 + 1.5x
4x - 1.5x > 150 + 1.5x - 1.5x
2.5x > 150
x > 150 ÷ 2.5
x > 60
To make Val the cheaper option, he needs to buy more than 60 bags of lettuces.