You want to spend less than $20 grapes and bananas. grapes are $3.00 per pound and bananas are $.50 per pound. let a represent the weight of the grapes and b represent the weight of the bananas.

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