Plenty of work is done in schools on pairs of numbers that make 10, but less is done for pairs that make 20. If children know off by heart pairs that make 10 then they should be able to quickly calculate what is needed to make 20.
A number line is included on the first page but the next three pages do not have any help. The last page is a more open ended investigation and it is always interesting to see how children tackle this type of question. Some will work haphazardly, often repeating themselves whilst others will work in a well organised, systemmatic way (eg 20 + 0 = 20, 19 + 1 = 20, 18 + 2 = 20 etc.) This could lead to an interesting discussion as to whether 13 + 7 is the same as 7 + 13 etc.