$8\mathrm{.}19.$ For the following data, caculate the mean absolute deviation.

Period

Forecast

Acutal Demand

Absolte Deviation

$1$

$100$

$105$

$2$

$105$

$95$

$3$

$110$

$90$

$4$

$123$

$130$

$5$

$120$

$100$

$6$

$125$

$120$

total

$675$

$650$