Answer 4) 5 is the correct answer.

 Follow the algebra: Isolate the unknown variable (X) and solve

Y = 2X +10  

-10       -10

 Y - 10 = 2X

(Y - 10)/2 = (2X)/2

(Y - 10)/2 = X

Now Input the values

(20 - 10)/2 = X

10/2 = X

5 = X