n + (10 - n % 10)
How this works. The % operator evaluates to the remainder of the division (so 41 % 10 evaluates to 1, while 45 % 10 evaluates to 5). Subtracting that from 10 evaluates to how much how much you need to reach the next multiple.
The only issue is that this will turn 40 into 50. If you don’t want that, you would need to add a check to make sure it’s not already a multiple of 10.
if (n % 10)
n = n + (10 - n % 10);