Half-lifes Please help!?

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

Answer 1

1.3 mole has a number of atoms

1.3* avogadro's number = 1.3*6.02*10^23

so No = 7.82610^23

27.8 days = 27.8*60*24 =40032minutes

you can use as dt 18 minutes is small the formula

dn = - ln2/40032*7.82610^23=1.08 10^20des