N(t) = N(0)*exp(-L*t)

where N(0) is the quantity of radioactive material present initially

N(t) is the quantity of radioactive material remaining at time t

L is the radioactive decay constant = ln(2)/halflife

The number of decays that have happened between times t=0 and t=t is then N(0) - N(t), so

# decays = N(0)*(1-exp(-L*t))

In this case, L = ln(2)/67 hrs = 1.724*10^-1 min^-1

N(0) = 1.4 moles

# decays = 1.4 moles * (1 - exp(-1.724*10^-1 min^-1 * 17 min))

# decays = 4.098*10^-3 mol

Multiplying by Avogadro’s number to get the number of actual decays:

# decays = 4.098*10^-3 mol* * 6.022*10^23 atoms/mol

# decays = 2.468*10^21 atoms decayed