Ok, so firstly let's think about what the mean means? Add all the numbers together and then divide by the total number of values we have.So, that means with the 4 numbers we currently have the total divided by 4 = 8.( ?+ ?+ ?+?) / 4 = 8 That means the total at the moment is 8 x 4 which = 32. We don't need to know what the individual are, but the total is very important and will help us answer the second part of this question. So, now we have the total , 32 we can think about how we can create an equation for the mean when we have 5 numbers:(32 + x)/5= 10 therefore if we multiply both sides by 5.. we will get the equation 32+ x = 50 therefore if we subtract 32 from both sides x = 18