What is the difference between rounding and using benchmark numbers

You deposit $5000 each year into an account earning 3% interest compounded annually. how much will you have in the account in 30 years?

Scores on the cognitive scale of the bayley-scales of infant and toddler development range from 1 to 19, with a standard deviation of 3. calculate the margin of error in estimating the true mean cognitive scale score, with 99% confidence, if there are 12 infants and toddlers in a sample.

Evan walked 5/8 mile to his friends house.then together they walked 7/12 mile to the movie theatre.which pair of fraction can evan use to find how far he walked in all?