Answer:
4
Explanation:
You need to try out all the numbers.
lets start with 4
if there where 4 red marbles then there would be 9 times as many marbles
so 4 x 9 = 36
Now we add and see if the sum is 40
36 + 4 = 40
We got it on the first try
You can use the same method on more problems like these.
Hope this helps :)