Add digit by digit, from the right, just like any number, except that if it adds to 2, then put a zero and carry one (instead of carrying when it adds to 10 or more).
Example: < means carry, decimal equivalent for checking
1011+1111
1 0 1 1 (8+2+1=11)
+ 1 1 1 1 (8+4+2+1=15)
---<---<----<----<----
1 1 0 1 0 (16+8+2=26)
Proceeding similarly,
a. 10101111+11011011 = 110001010 (394)
b. 10010111+11111111 = 110010110 (406)
c. 01110101+10101100 = 10010001 (289)