Answer: 
Step-by-step explanation:
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The factors of 13 are 1 and 13 .
13 is a "prime" number.
To solve for the x, distribute first the numerical coefficients to the terms on the parentheses. This becomes,
7x - 21 = 4x + 20
Transpose all the terms with x and the constant on either sides of the equation,
7x - 4x = 20 + 21
3x = 41 ; x = 41 / 3
Thus, the answer is x = 41/3 which is fourth among the choices.
The average will be the third number, because the first and fifth, as well as the second and fourth, will average out to the third.
