I didn't know you can put a function there! So cool.
Elegant solution.

The testcase Test.assertEquals(splitTheBill(100, 2, 2), 50,); should be Test.assertEquals(splitTheBill(100, 2, 2), 50);, without the , after 50.

Add some edge cases tests.
Add random tests.

I'm new to this - can you let me know how you think they should be improved? Thanks!

Thank you for pointing this out! It should be 'round' and I will amend it shortly.

poor tests

Keep the values to 2 decimal places.

How to keep 2 decimal places? round, floor, or ceil?

I used ceil, because in testcase1:

splitTheBill(100, 2, 1), '45.45,54.54'

I got two decimal numbers:

45.45454545454545 54.54545454545454

But I failed at testcase3:

Test.assertEquals(splitTheBill(59, 4, 1), '14.05,16.86');

I got two decimal numbers:

14.047619047619047 16.857142857142858

Expected: '14.05,16.86', instead got: '14.04,16.85'