if 40 - x < 3x, then the solution
a. x < 20
b. x > 20
c. x > 10
d. x < 10
!

40 - x < 3x

Simplify both sides of inequality:

−x + 40 < 3x
Subtract 3x on both sides:

−x + 40 − 3x < 3x − 3x−4x + 40 < 0

Divide both sides by -4:

-4x/-4 < -40/-4

x > 10

C. x > 10

Solutions for  40 - x < 3x

To find the answer we have to solve the equation 40 - x < 3x. 

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1) The first step is to add x to each side 
40 < 3x+x

2) Next you simplify 3x+x. To simplify 3x+x you add the x to 3. 3x+x = 4x  
40 < 4x

3) Divide both of the sides by 4
 < x

4) Simplify the fraction  to 10. You simplify it to ten because 40 ÷ 4 = 10.
10<x

5) Answer 
x= > 10 

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Now lets see the Choices 
The correct choice is C. 

x > 10