Ed has 4 cards. There is a number on each card. Three of the numbers are 12, 6 and 15. The mean of the numbers is 10. What is the fourth number?

To find the mean you add all the numbers together and divide by the number of numbers. Call the fourth number x. Then we know that (12+6+15+x)/4 =10. then multiply both sides by 4 to get 12+6+15+x=40 which is 33+x=40. And subtracting 33 from 40 gives x=7. The fourth number is 7.