Half Life help please?

Question

For 1.3 mole of the radioactive isotope chromium-51, half-life = 27.8 days, how many disintegrations have occurred after 18 minutes?

Enter a numeric answer only do not include units

Note: The answer is not 1.08e20

Answer 1

M = Mo(1/2)^t/27.8, t in days
27.8 days = 27.8*24*60 = 40,032 min.
∆M = Mo - M = Mo(1 - (1/2)^18/40,032)
∆M = 4.051038e-4 moles
N = 6.02214179e+23 atoms/mole
N∆M = 2.439593e+20 disintegrations