The sum of 24.686 m, 2.343 m and 3.21 m with correct significant figures is

30.24m

Explanation:

When adding numbers together, we round to the smallest number of decimal places of the numbers, which is 2 decimal places in this case.

24.686m + 2.343m + 3.21m = 30.239 m

                                             ≈ 30.24m (round to 2 d.p.)

When you round numbers to a certain digit (hundredth in this case), you would want to look on the digit on its right (thousandth in this case). If that digit is 1-4, you round the digit down. If that digit is 5-9, you round the digit up. Since the digit in the thousandths place is 9, you round the hundredth digit up from 3 to 4.

since you're a real machine, your ama is always less than your ima, because you can never totally eliminate friction.