Answer:
Martin paints walls at a rate of 1.28 walls per Gallon, or 6.24/5 Walls Per Gallon.
Explanation:
First, we need to find a common denominator between the two values stated (being the amount of paint Martin uses, and the amount of the wall he covers). To find this, we multiply both the numerator and denominator of each fraction by the opposite fraction's original denominator. This gives us two new fractions; 25/40, and 32/40. This means Martin uses 25/40, or 62.5% of a Gallon to paint 80% of a wall. With 20% of the wall left unpainted with this much paint, we must divide the amount used by four, as this will determine how much paint Martin uses to paint 20% of a wall. This gives us 15.625%, or 6.25/40 for 20% of a wall. Thus, Martin needs 95.625%, or 38.25/40 of a gallon per wall. This leaves us with 1.75/40 remaining to achieve our 1 gallon necessary. By repeating our earlier calculations (dividing both terms down to 4.375% of their value) we can calculate that Martin can paint 1.28 walls per Gallon.