Problem: 23​+45​+89​+1617​+3233​+6465​−7=
Answer Choices:
A. −641​
B. −161​
C. 0
D. 161​
E. 641​
Solution:
Add and subtract directly or note that
23​+45​+⋯+6465​−7​=(1+21​)+(1+41​)+⋯+(1+641​)−7=21​+41​+⋯+641​−1=−641​​