ⁿP _r=240 এবং ⁿC_r=120 হয়, তবে n ও r এর মান নির্ণয় করো। 

${ }^{\mathrm{n}} \mathrm{P}_{\mathrm{r}}=\frac{\mathrm{n} !}{(\mathrm{n}-\mathrm{r}) !}=240$ and ${ }^{\mathrm{n}} \mathrm{C}_{\mathrm{r}}=\frac{\mathrm{n} !}{\mathrm{r} !(\mathrm{n}-\mathrm{r}) !}=120$

$\mathrm{r} !=2 \text { or } \mathrm{r}=2 \quad \therefore \mathrm{n}^{2}-\mathrm{n}=240 \quad \therefore(\mathrm{n}-16)(\mathrm{n}+15)=0 \quad \therefore \mathrm{n}=16$