Three numbers have a mean of 64. when a fourth number is included the mean is doubled. what is the fourth number?

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

The missing number is 320..

Now lets double check our work.

Mean=(64+64+64+320)/4=512/4=128

Answer=320