This is one of those "can you spot the short way" type questions. DebsB has wonderfully given the short way to do the second and third questions, using the answer they have already given for s(6,39).
And in the final question, they're looking to see if you spot that most of the terms cancel each other out:
s(1,2) - s(2,3) + s(3,4) - s(4,5)+ ......... - s(18,19) + s(19,20)
= 1 + 2 - 2 - 3 + 3 + 4 - 4 - 5 + ... - 18 - 19 + 19 + 20
= 1 + 20
Nicely done, rah_b92. I hadn't even noticed that the final question was there.
Shocking example of failure to read the question properly - let's hope none of our kids have been throwing marks away by missing stuff out in that way!