The mean is calculated by adding up all of the given numbers and dividing by the number of numbers.
Lets, for ease of understanding, say that the first three number were 64...
Mean=(64+64+64)/3=192/3=64
Now, lets solve for the mean where 'x' is the unknown fourth number and 128 is the mean. (since, the answer said that when the fourth number was added the mean, 64, was doubled '64*2=128')
128=(64+64+64+x)/4 128=(192+x)/4 Multiply both sides by 4 512=192+x subtract 192 from both sides 320=x
so first of all, if the mean is 64 then, u divide the 64/2 and u will get 32 so 32 + 32 would give u 64 but u wanna get the fourth number which 16 so then, u divide 32/2 which gets u the number 16 and then if u add up 16 + 16 + 16 + 16 = 64 so now 16 is the fourth number.
im sincerely sorry cuz i didn't know any other way to explain it but i hope this is correct!! :)
