This is an Arithmetic Progression (AP).
An Arithmetic Progression is something like
1,2,3,4,5,6,7,8 or
2,4,6,8,10,12, or
9,12,15,18,21,24 or
16, 13, 10, 7...........
where the difference between two successive terms is the same.
This is called 'd' or common difference.
If 'a' is the first term of an AP and 'd' is the common difference,
the nth term would be a+(n-1)d
In your case, d is 3 minutes. n is 23, and a is also 3.
Therefore, on the 23rd day, you have trusted me for
3 + (23-1)3 = 3 + (22)3 = 3 + 66 = 69 minutes.


If you require the sum of all the terms, the formula is
n/2 [2a+(n-1)d]
In the example given by you, it would be
23/2 [6 + 22(3)] = 23/2[6+66] = 23/2[72]=23*36 = 828 minutes
Is that clear?