Mary filled a water cooler with 6 1\2 gallons of water. she forgot to close the plug and 2 5\6 gallons leaked out. she adds 1/4 gallons how many gallons are there now

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.