To clarify you cannot add zeros to a non-convergent series, which the series c is.

In regular summation you are only allowed to add one zero to the start of a convergent series without changing it's value, since you know a convergent series has a specific answer.

But for non-convergent series you cannot do this mathematically in normal summation.

The value of a series is calculated by summing to n digits, and extrapolating. So c to 4 digits is 10, and to 5 digits is 15. 4c to 4 digits is 40, and to 5 digits is 60. But the series 4c with added zeroes at 4 digits is 12, and at 5 digits is still 12.

So 4c and 4c plus zeroes are not the same series. The only way to make 4c work in the posted equation is to use "super summation" which is a load of bull. Someone else posted a good video showing why this is the case.