Question 52008
The key to multiplying rational expressions is to factor and cancel.  I think you meant to solve:
(3x-30)/(x^2-4x)*(8x)/(10-x)
(3(x-10))/(x(x-4))*(8x)/(-1(-10+x))   Factor the numerators and denominators.
(3(x-10))/(x(x-4))*(8x)/(-1(x-10))    
(24x(x-10))/(-x(x-4)(x-10))  Multiply numerators times numerators and denominators times denominators.
-24/(x-4) Cancel any numerators and denominators that match.