Answer :

Sada
the only numbers that end in 0 when you double them have to end in 5. 

and there's only two numbers greater than 15 and less that 40 that end in 5: 25 and 35 

now it's trial and error. 

does 1/5 of 25 equal 5?? yes 

does 1/5 of 35 equal 5?? no 

therefore the answer must be 25 

hope this helps!