Question

The sum of two numbers is 70, and their difference 30. Find the two numbers using the process of substitution. Let the first umber = x and the second number = y.

Answer 1

x = 50, y = 20.

Explanation:

x + y = 70

x - y = 30

From the second equation x = 30 + y, so substituting this in the first equation:

30 + y + y = 70

2y = 70 - 30 = 40

y = 20

and x - 20 = 30

so x = 50.

Answer 2

20 , 50

Explanation:

The sum of two numbers is 70 ⇒ x + y = 70 -------------(I)

x = 70 - y

Their difference 30 ⇒ x - y = 30 ----------(II)

Substitute x = 70 - y in equation (II)

70 - y - y = 30

Combine like terms

70 - 2y = 30

-2y = 30 - 70

-2y = -40

Divide both sides by (-2)

y = -40/-2

x = 20

Substitute the value of x in (I)

20 + y = 70

y = 70 - 20

y = 50