Rounding 43 to the nearest tenth would get you 40. You look at whether it is closer to 40 or 50 because the 4 is in the tenths place. Since the one's place is less than 5, you round down. The answer is 40.
You do you calculation. Let's say the answer is $7.4269. It means seven dollars, 42 cents and .69 of a cent. The nearest whole cent is 43, not 42. So you write $7.43. If you round that to the nearest dollar, it will be $7. Forget about the change.
When estimating sums, it is best to round to the nearest round number. If the last place on the right is less than 5, then round down. If the last place on the right is 5 or more, then round up. For example, 43+88. You would round the 43 down to 40 and the 88 up to 90. The estimated sum would 130.