If Avi takes x hours to make 1 basket then it takes Markus x - 0.5 hours.
Avi makes 1/x baskets in 1 hour and Markus makes 1/(x - 0.5) baskets in 1 hour.
so we have the equation:-
28 /( x - 0.5) - 28/x = 1
28x - 28(x - 0.5) = x(x - 0.5)
x^2 - 0.5x -14 = 0
x = 4 , -3.5 (ignore the negative)
x = 4
It takes Avi 4 hours to make one basket.