Let’s use m for months
So,
1st service
10m + 20
1. 10 + 20 =30
2. 10x2=20+20=40
3.10x3=30+20=50
4.10x4=40+20=60
5.10x5=50+20=70
2nd service
11m + 15
1. 11+15=26
2. 11x2=22+15=37
3 11x3=33+15=48
4.11x4=44+15=59
5.11x5=55+15=70
So, the 5th month the services cost the same
Hope this helps! ;)