You want to replace the countertops in a kitchen. one counter measures 24 inches by 6 feet. the other is 30 inches by 4 feet. if the new countertop costs $42.00 per square foot, what will be the cost of the new top?

First of all you have two of them. Convert all inches to feet.
1 foot = 12 inches
2 feet = 24 inches.
30 inches is a little harder
30/12 = 2.5 feet. Now you are ready to figure the problem
Top A

Area = L * w
L = 6 feet
w = 2 feet
A = 2 * 6 = 12 Square feet
Top B

Area = 2.5 * 4
Area = 10 square Feet

Total Area = 10 + 12= 22 square feet

Cost 

Square feet * 42
Cost = 22 * 42
Cost = $924