Dennis can load the car in 28 min. if andy him it takes 21 min to load the car how long will it take andy alone?

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.

Yes, that is the correct formula. to solve this problem given the number of years, plug in the number of years for t and plug the whole thing into the