whenever two numbers vary whether it be inversely, jointly, directly, etc. there is always a constant value k to solve for.
u = k/t
the first info provides enough to solve for k.
25 = k/5
25*5 = k
125 = k
now you have the equation
u = 125/t
1.) when t = 50
u = 125/50
u = 5/2
2.) when u = 10
10 = 125/t
t = 125/10
t = 61/2
