Can I solve the problem by subtracting 81 from 358 and dividing the resulting 277 by 5 to get 55?
Your logic is sound. And it was correct not to round up to 56 when you found 55.4—but make sure you check that you are rounding correctly when you do this.
You can check whether your rounding up or down is correct by using known values.
Let's say that we have a set of integers from 1 - 27. How many of those numbers are divisible by 5?
26/5 = 5.2
Actual numbers: 5, 10 , 15, 20, 25 ---> 5 numbers
So we round down 5.2 to get the correct value, so we know that rounding 55.4 down to 55 should be correct.