Compute each of the following mentally and describe an approach that is valid.

a$.206-91$

b$.25-40+175-50$

a$.206-91=$

Choose the best description of a good process to make the calculation easier.

A$.$ Add a value to $91$ to make it equal $100$ and add the same value to $206.$

B$.$ Add a value to $91$ to make it equal $100$ and subtract the same value from $206.$

C$.$ Subtract a value from $206$ to make it an even multiple of $100$ and add the same value to $91.$

D$.$ Subtract a value from $206$ to make it an even multiple of $100$ and subtract the same value from $91.$

b$.25-40+175-50=$

Choose the best description of a good process to make the calculation easier.

A$.$ Add the compatible numbers $25$ and $40$ and then subtract.

B$.$ Add the compatible numbers $25$ and $175$ and then subtract.

C$.$ Add the compatible numbers $40$ and $175$ and then subtract.

D$.$ Add the compatible numbers $175$ and $50$ and then subtract.