Half-life is defined as the amount of time it takes a given quantity to decrease to half of its initial value. The formula that describe the exponential decay is: N(t)=N0 (1/2)∧(t/t(1/2)) where N0 is the initial quantity Nt is the remaining quantity after time, t t(1/2) is the half-life
So, t(1/2) = (-㏑2)*t/㏑(Nt/N0) = (-㏑2)*10/㏑((100-75)/100) = 5 years. The half-life of this isotope is 5 years.