In order to find the average of a set of numbers you have to add all the numbers given and then divide the value that you get by the amount of numbers added. In the question you were given 7 numbers therefore, you add all of then and then divide the value you get by 7.
Question2
To calculate the mean we do as we did above and add all the numbers given then divide the value obtained by the amount of numbers added. However in this question we are missing a number so we substitute the value and call it *x*. So, (5+7+7+x)/ 4 = 7. we cross multiply to get 4x7 which is 28. The new equation would then be: (5+7+7+x) =28, you then group the like terms to get: 5+7+7-28 = x. Therefore x is equal to 9. Meaning that the forth number is *9*
