Too lazy..hmmmm

Find moles of helium from data given P,V,R,T now this is six times the disintegrated part from thorium,

So find the disintegrated part("a"),Now the left over(undecomposed) number of moles is also given

To find initial: undecomposed number of moles(5*10^-7/232) + a.

Use this in the formula:)

If you still want the entire process tell.