Lets say the cost of her haircut is $x. This makes the cost of her perm 2x (twice as much). If lunch ($5), the perm ($2x) and the haircut ($x) cost, in total, $50, we can say:
50 = 5 + 2x + x Combine like terms: 50 = 5 + 3x Subtract 5 from both sides to isolate 3x 50 - 5 = 3x 45 = 3x Divide all by 3 to isolate x x = 45/3 x = 15
So the haircut is $15 and the perm is $30 (2*$15).
You substitute the given value of y into the equation 8x+4(3x+8)=12 Solve the equation for x x= -1 Substitute the given value of x into the equation y=3x+8 y=3x (-1)+8 Solve the equation for y y=5