Joe is trying to soup up his dragster He knows that the time needed for the car to go from 0 to 100 miles per hour varies inversely with the cars horsepower At 200 horsepower the car can go from 0 to 100 mph in 10 seconds How long should the car?

The question appears incomplete but certain facts can be deduced.
As Time (T) varies inversely with Horsepower (P) then, T ∝ 1/P which implies T = k/P where k is a constant.
When T = 10, P = 200 therefore , 10 = k/200 : K = 200 x 10 = 2000
The formula now is, T = 2000/P
By substituting the appropriate figure(s) in this formula either Time or Horsepower can be calculated.