Mr. Ross is painting a wall that has an area of 142.5 square feet. He uses 3/4 gallon of paint to paint 1/5 of the wall. How many gallons of paint will he need to paint the entire wall?

I don't care what the area of the whole wall is, because we don't need it to solve this problem.  It's just there to confuse us.

==>  If he used 3/4 of a gallon to paint 1/5 of the wall, then in order to paint the whole wall, he'll have to use 3/4 of a gallon FIVE TIMES.

(5) x (3/4 gallon) = (15/4 gallon)  = 3-3/4 gallons

Solving this problem may be painful, but it's not difficult.