Number of atoms of He in 100 amu of He (atomic wt. of He is 4) are

1 amu = 1.66 x 10^-24 g
100 amu = 100 x 1.66 x 10^-24 gm
No. of atoms of a substance
$=\frac{\text{ Wt. of substance }}{\text{ No. of gm moles }}\times 6.023\times {10}^{23}\times \text{ Atomicity }$
$=\frac{100\times 1.66\times {10}^{-24}}{4}\times 6.023\times {10}^{23}\times 1=25$