There are 10 boys and 20 girls in a class. The mean mark in a test for all the class is 60. The mean mark for the girls is 54. Work out the mean mark for the boys.

First let us consider how we calculate the mean test mark in the first place, we add up the individual test marks of the entire class to give the total test mark then we divide by the number of students. This means we need two pieces of information to calculate the boys' mean mark, the total boys' mark and the number of boys. We already know the number of boys but how do we work out the total boys' test mark? If we can work out the total class mark and total girls' mark we can find the total boys' mark by subtracting the total girls' mark from the total class mark. To do this we can work backwards to find the total mark for the whole class by taking the mean mark and multiplying it by the number of students, this gives us Total Class Mark = Mean Class Mark x Number of Students = 60 x 30 = 1800. We can do the same for the girls' mark giving us Total Girls' Mark = Mean Girls' Mark x Number of Girls = 54 x 20 = 1080. This then gives us the total boys mark as Total Boys' Mark = Total Class Mark - Total Girls' Mark = 1800 - 1080 = 720. Now we have everything we need to work out the boys' mean mark and we get Boys' Mean Mark = Total Boys' Mark/Number of Boys = 720/10 = 72.