If the variance of the frequency distribution

| $x$ | $c$ | $2c$ | $3c$ | $4c$ | $5c$ | $6c$ |
|-|-|-|-|-|-|-|
| $f$ | $2$ | $1$ | $1$ | $1$ | $1$ | $1$ |

is $160$, then the value of $c \in \mathbb{N}$ is