Out of 15000 lettuces n lettuces are unsuccessful and can’t be sold. Let c=cost of producing one lettuce (successful or unsuccessful). Total cost of producing all lettuces=15000c. No fixed costs are mentioned. If s=sale price of 1 lettuce, income is (15000-n)s.

Total profit = (15000-n)s-15000c=15000s-ns-15000c=15000(s-c)-ns.

s-c=R1.72 for a successful lettuce, so total profit=25800-ns=15380. Therefore ns=R10420. This is the total of lost sales because of unsuccessful lettuces. We don’t know the sale price of a lettuce. If s=R5, then n=2084, if s=R4, n=2605, if s=R10, n=1042, if s=R20, n=521. If we assume n=521, then 521/15000 are unsuccessful, and this means that in 20 lettuces, less than 1 would be unsuccessful.

Now assume n=1042, then about 1.39 in 20 are unsuccessful; n=2605, 3.47 unsuccessful in 20; n=2084, 2.78 unsuccessful in 20.