I have to disagree recursion is unecessary and clunky. What should be done is repeating a block of code n times, this is achieved by a loop of any kind not recursion. If you unroll above code you have alot of redundancy (null check and creating StringBuilder) and absolutely no benefit.

Using substring that often is very expensive.

With v1=820, v2=850, and g=550
I get mins 19,99999999999993, so I return 19 (not rounded) and it happens to expect 20.

The correct number of minutes in this case is exactly 20, not less; no problem with the tests here.

Was this problem solved?
I'm trying to solve this kata in python, but I ended with the same problem.