Find the half-life (in hours) of a radioactive substance that is reduced by 25 percent in 100 hours.

The half-life of a certain substance is the time by which its amount decreases to half of the original amount. By the given above,

0.25A = A (1/2) ^ (100/h)

where h is the half-life. Simplifying,

0.25 = 0.5^ (100/h)

The value of h from the equation is 50 years.