Question

If 7x - 4y = 23 and x + y = 8, what is the value of x?

Answer 1

`\huge\boxed{x=5}`

Explanation:

`\left\{\begin{array}{ccc}7x-4y=23\\x+y=8&|\text{subtract}\ x\ \text{from both sides}\end{array}\right\\\left\{\begin{array}{ccc}7x-4y=23&(1)\\y=8-x&(2)\end{array}\right\\\\\text{substitute (2) to (1):}\\\\7x-4(8-x)=23\qquad|\text{use the distributive property}\\\\7x+(-4)(8)+(-4)(-x)=23\\\\7x-32+4x=23\qquad|\text{combine like terms}\\\\(7x+4x)-32=23\qquad|\text{add 32 to both sides}\\\\11x-32+32=23+32\\\\11x=55\qquad|\text{divide both sides by 11}\\\\(11x)/(11)=(55)/(11)\\\\x=5`

Answer 2

x=5

Explanation:

7x - 4y = 23 and x + y = 8

Multiply the second equation by 4

4x + 4y = 32

Add this to the first equation

7x - 4y = 23

4x + 4y = 32

------------------------

11x = 55

Divide each side by 11

11x/11 = 55/11

x = 5