In this case, we'll have to carry out several steps to find the solution. 
Step 01:
Data
solve the riddle = ? 
Step 02:
We must analyze the exercise to find the solution
a. even number 
b. 10 ----- 20 
c. x / 7 ===> no remainder 
11 , 13 , 15 , 17 , 19 ===> even numbers