We can solve with a proportion
12 / [ 18 + 3/4 ] = M / [ 28 + 1/8 ] cross-multiply
12 [ 28 + 1/8 ] = [ 18 + 3/4 ] M
12 [ 225 / 8 ] = [75 / 4 ] M mutiply through by 8
12 * 225 = 150 M
2700 = 150 M divide both sides by 150
18 = M ⇒ 18 motors in 28 + 1/8 hrs
