3. We can see that
1+100=101
2+99=101
3+98=101
.................
and so on
From 1 to 100, there are 50 pairs that has the sum is 101
Z=101×50=5050
4. We can see that
1-2=-1
3-4=-1
5-6=-1
...........
and so on
There are 10 pairs that has sum is -1
Z= 10×(-1)=-10