# How many moles are present in 2.1 * 10^24 atoms of calcium?

There are $6.022140857 \left(74\right) \times {10}^{23}$ $\text{calcium}$ $\text{atoms}$ in 1 mole of $C a$.
You have specified $2.1 \times {10}^{24}$ calcium atoms.
So $\left(2.1 \times {10}^{24} \cdot {\text{calcium atoms")/(6.022xx10^(23)*"calcium atoms mol}}^{-} 1\right)$ $=$ ?? $m o l$