Answer the next $2$ questions using the following marginal benefit $($MB$)$ and marginal cost $($MC$)$ functions for activity $A$ :

$MB=400-20$A and $MC=-50+10A$

$1-$ The optimal level of activity $A^{**}$ is

$2-$ Undertaking the ${18}^{th}$ unit of the activity A will

Net Benefit by