Answer the next $6$ questions using the following marginal benefit $($MB$)$ and marginal cost $($MC$)$ functions for activity $A$ :

$MB=1200-20$A and $MC=500+30A$

and the following total benefit $($TB$)$ and total cost $($TC$)$ functions:

$TB=1200A-10A^2$ and $TC=500A+15A^2$

$1-$ Undertaking the ${10}^{th}$ unit of the activity $A$

Net Benefit by

$2-$ Undertaking the ${20}^{th}$ unit of the activity $A$

Net Benefit by

$3-$ The optimal level of activity $A^{**}$ is

$4-$ At the optimal activity level of $A^{**},$ Net Benefit is equal to

$5-$ Eliminating the ${25}^{th}$ unit of the activity Total Benefit by and

Total Cost by

$6-$ What level of activity maximizes TOTAL Benefit?