Alright so let's start this off simply. You know there are 5 values which are 102, 121, 106, 111, and 107 and there is an unknown x value. You know that mean is 100 and to find it you must add all the numbers and then divide it by how many of those numbers are there. So you should do 102+121+106+111+107+x and you know there are 6 of these values here including x. So you should get (547 + x)/6 = 100. Then just multiply the 6 on both sides. So the 6 on the left side cancels out and 6x100 is 600. So now you have 547+x=600. Now you just subtract 547 and you get x=53. Happy to help!
