Question

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

Answer 1

Answer:

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

Answer 2

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):

$\frac{138+40+102+x}{4}=90$

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

$\frac{280+x}{4}=90$

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.