 In this video we are going to take a look at finding a missing value when you've been given the mean. Mean alongside mode and median is a form of average and is found by dividing the sum total of values by the number of values themselves. This is sometimes referred to as sum over count. The application of this process, division, can be reversed to help us find an original value given the mean. The reverse process to division is of course, multiplication. Consider this scenario, 30 students sit a maths test, 10 boys, 20 girls. The girls marks total 320, the boys marks total 130. Calculate the mean for boys and girls and the class. Pause if you want to give it a go. Looking at how we tackle this problem will help us see how to reverse it. By putting the values into a table like this, we can use the mean equation to work out the individual means. We can similarly use this table to help us find initial unknown sums or counts. If you want to have a go, pause now to try. The best way for us to tackle this problem is to use the same table from the first question. This time however, we won't be filling in the sum column, but we can fill the mean column, like so. At this stage, we have no sum values or information for the girls, but we can work this out given the information in the table. As there are 50 people in total and 30 boys, there must be 20 girls. We now need to work out the sum marks for the total students and the boys. Because we divide to find the mean, the reverse should give us the sum, so we multiply the mean of each by their count. We can put this information into our table. Because the total sum marks come from the sum of the boys and the girls marks, we can subtract the boys to find the girls' total marks and the first part of our answer, 172.5. Finding the mean is then a simple matter of sum divided by count, for a mean of 8.675. Using this method, try this question, let's see how you did. Putting the information into a table looks like this. Finding the sum points requires us to work backwards, so we multiply the mean by the count. We can see from this that the fused firecrackers scored 24 points in their 11th game. So there you have a guide to finding unknown sums. This method also counts for finding the number of people, the count. To do so, we use the sum value and mean and divide the mean in place of the count, like you can see here. By switching the mean and the count, a simple division lets us find the missing value. If you liked the video, give it a thumbs up and don't forget to subscribe, comment below if you have any questions. Why not check out our Fuse School app as well? Until next time.