p  n
=====
1  9
2  11
3  12
4  14
5  15
6  17
7  18
8  20
9  21
10 23
11 24
12 26
13 27
14 29
15 30
16 32
17 33
18 35
19 36
20 38

At least n exercise questions have to be solved to get p points.