Question

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

Answer 1

Steps

So remember that the average formula is
`\frac{\textsf{sum of all the numbers}}{\textsf{how many numbers there are}}` . With the info that we are given, our equation is (Let x = fourth number):

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

From here we can solve for x. Firstly, add up the numerator:

`(280+x)/(4)=90`

Next, multiply both sides by 4:

`280+x=360`

Lastly, subtract both sides by 280:

`x=80`

Answer

In short, the fourth number is 80.

Answer 2

x=80

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