let the load car = y , and Andy will take time x (min.) to load the car So, The rate of Andy to load the car = y/x
And we given that: Dennis load the car in 28 min. So, The rate of Dennis to load the car = y/28
If both are loading the car in time = t = 21 min then: y = (y/x) t + (y/28) t ⇒ divide all equation by y and substitute with t ∴ 1 = (21/x) + (21/28) ⇒ multiply all equation by 28x 28x = 21*28 + 21x 28x - 21 x = 588 7x = 588 ∴ x = 84 min.
Which mean Andy will take 84 min. to load the car alone.
