Two times a number plus 6 equals 20

First subtract 6 from 20, then you can deduce what the other number is. Seven times two is 14, so we know the missing number is 7.

You could also write it as the algebraic expression 2x + 6 = 20
2x+6=20
-6       -6
2x=14
14/2=7
x = 7  

7x the number of visitorsex. 3 visitors would be 7 x 3 =21$ total