X varies directly as the product of u and v and inversely as their sum. If x=3 when u=3 and v =1, what is the value of x if u=3 and v=3
x = kuv/(u+v)
That means that x(u+v)/(uv) = k, a constant. So, you want x such that
x(3+3)/(3*3) = 3(3+1)/(3*1)
x=6
Note that you don't have to evaluate k to solve the problem.