If the average of two numbers is x+y, and one of the numbers is x, what is the other number?

Question

 I need help with this.

Answer 1

average = ½(A+B) = x+y 

Now, let's say that A=x

i.e.  ½(x+B) = x+y 

 

Solve for B, the 'other number:  (x+B) = 2x+2y

B = x + 2y

 

Answer 2

Hi Jeremy,

 

The average of two numbers a and b is (a + b)/2

If I told you the average was (a + b)/2, and the first number was a you would do the following:

 

((a + b)/2)*2 = (a + b) = the sum of my two numbers.

but one of the numbers is a, so my other number is (a + b)-a = b. My other number is b.

 

 

In this problem the average = x + y. Do the same as we did above.

 

(x+y)*2 = (2x + 2y) = the sum of my two numbers

Now subtract off the number you know, and you will have the other number.