If two people averaged 15 years old, how old are they added together?
remember, the average is: average=(the sum of all tries)/(the number of tries)
substitute: 15=(The sum of all tries)/2
multiply both sides by two to get: 30=The sum of all tries
but what if someone messed up and one of the people's ages were 4 years older, what would be the mean then?
We know that the sum is already 30, so all we need to do is add 4 on and find the new mean, so:
Average=(The sum of all tries)/(the number of tries)
Can you do your problem now?