If we say s is the cost per pound of strawberry and b is banana.
For Annie we have 9s + 10b = 103.
For Rob we have 9s + 9b = 99.
If you notice the only thing that changes on the left hand sides is a single b. This causes a change of 4 on the right hand side. From this we can see that b = 4. Substituting b=4 in Annie's line we get 9s + 40 = 103 so 9s = 63 so s = 7.
Strawberry costs $7 per pound and banana costs $4 per pound
