the answer
the main formula to solve such a problem is given by:
m = m0 x (1/2) ^t /T
where,
m is the amount of the mass after time t
m0 is the amount of the initial mass
T is the half-time found by means of the formula T = log 2 / λ = 22.3 years
at t= 0, m0 = 7.50 g, we can find the valuo of the mass after 17.5 years by applying the formula given above:
m = m0 x (1/2) ^t /T
m0 = 7.50 g, t= 17.5 years, T = 22.3 years
so m=7.50 x (0.5)^17.5 / 22.3=7.50 x (0.5)^0.78 = 4.36 g
