ok, I don't know any big trick for adding numbers under 100 but there are some easy to know shortcuts :

1) always go with the the multiples of ten : add them up.

2) Add the numbers in the units together.

3) Add 1) with 2)

ex.: 43 + 27 :

1) 40 + 20 = 60

2) 3 + 7 =10

3) 60 + 10 = 70

It seems long but if you get used to this it will be, I think, very fast because it is easier than adding directly 43 + 27