I know that 1 mole of gas at 0°C and 1 atm has a volume of 22.4 liters. So first, I'll convert the 21 liters of Helium at 1.15 atmospheres to x liters of Helium at 1 atm. For this, I will use the expression:

$$P_{1}V_{1}=P_{2}V_{2}$$

$$1.15*21=1*V_{2}$$

$$V_{2}=24.15$$

So, at 1 atm, we have 24.15 liters of gas. We can use a comparison:

$$\frac{n}{N_{a}}=\frac{24.15}{22.4}$$

Where n is the number of atoms for 24.15 liters of gas and N_{a} is the number of atoms for one mole (22.4 liters) of gas.

$$n=\frac{24.15}{22.4}*N_{a}$$

$$n≈6.49*10^{23}$$

Is this correct? Thank you in advance.