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

$MB=10-0\mathrm{.}05$A and $MC=2+0\mathrm{.}05A$

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

$TB=10A-0\mathrm{.}025A^2$ and $TC=2A+0\mathrm{.}025A^2.$

Undertaking the ${70}^{th}$ unit of the activity

Total Benefit by

At the optimal activity level of A$*,$ Net Benefit is equal to