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!
