consider a base 16 adder how to modify the adder so that it can perform a base 8 addition?
Assumptions/Statements:
A base 16 adder: an adder that adds two 4-bit binary numbers I1 and I2 and outputs the 4-bit result R and a carry bit C

To use as a base 8 adder (as above) discard the Most Significant Bit from both I1 and I2 (i.e. ground/zero them). Now your inputs are base-8. From the output discard the Carry bit C it will always be zero. Your new Carry bit will be the Most Significant Bit from result R. Your new result will be the 3 least significant bits from R.

Please check Assumptions/Statements are in accordance to your book!