Grapes are $3.00 per lb. a is the number of pounds of grapes.
The cost of grapes is 3a.
Bananas are $0.50 per lb. b is the number of pounds of grapes.
The cost of bananas is 0.5a.
The cost of grapes and bananas is 3a + 0.5b
You want the cost to be less than $20, so the inequality is
3a + 0.5b < 20