The average of four numbers is 90.three of them are 138,40,and 102 find the fourth number

x=80

Step-by-step explanation:

To find the average, we take the numbers, add them up.  Then divide by the number of terms we added.

Let x= the unknown number

The average =90

Average = (138+40+102+x)/4

90 = (138+40+102+x)/4

Multiply by 4 on each side

90*4 =(138+40+102+x)/4 *4

360 = (138+40+102+x)

Combine like terms

360 = 280 +x

Subtract 280 from each side

360-280 = 280+x-280

80 =x