"The sum of two numbers is 20" can be translated mathematically into the equation:
x + y = 20.
"... and their difference is 10" can be translated mathematically as:
x - y = 10
We can now find the two unknown numbers, x and y, because we now have a system of two equations in two unknowns, x and y. We'll use the Addition-Subtraction Method, also know as the Elimination Method, to solve this system of equations for x and y by first eliminating one of the variables, y, by adding the second equation to the first equation to get a third equation in just one unknown, x, as follows:
Adding the two equations will eliminate the variable y:
x + y = 20
x - y = 10
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2x + 0 = 30
2x = 30
(2x)/2 = 30/2
(2/2)x = 15
(1)x = 15
x = 15
Now, substitute x = 15 back into one of the two original equations. Let's use the equation showing the sum of x and y as follows (Note: We could have used the other equation instead):
x + y = 20
15 + y = 20
15 - 15 + y = 20 - 15
0 + y = 5
y = 5
CHECK:
In order for x = 15 and y = 5 to be the solution to our original system of two linear equations in two unknowns, x and y, this pair of numbers must satisfy BOTH equations as follows:
x + y = 20 x - y = 10
15 + 5 = 20 15 - 5 = 10
20 = 20 10 = 10
Therefore, x = 15 and y = 5 is indeed the solution to our original system of two linear equations in two unknowns, x and y, and the product of the two numbers x = 15 and y = 5 is:
xy = 15(5)
xy = 75
