A washer machine is filling up with water at a constant rate 3/4 of a gallon and 1/5 of a minute. Don calculated the unit rate to be 3/20 of a gallon per minute what did he do wrong

The unit rate would be

(3/4 gal) / (1/5 min)

because the machine is filled with 3/4 gal of water in 1/5 of a minute. Do the arithmetic to get the unit rate of

(3/4 gal) / (1/5 min) = (3/4 • 5) gal/min = 15/4 gal/min

Don's mistake was to multiply 3/4 by 1/5 to get 3/(4•5) = 3/20, but he should have divided by 1/5 instead.