First lets start by making the denominators the same. I multiplied the 4 by 5 and 10 by 2.
-25/20 = x -14/20
Add 14/20 on both sides (do the opposite to cancel out and whatever you do to one side you need to do to the other.
You should get -11/20 = x
If you change both to a fraction you would see that you have 1/6 and 1/4 (.25 as a fraction). So 1/5 would be in between them.
You could also change both as a decimal. 1/6 =.166666 and .25 so any number in between them would be correct such as .2 which is the same as 1/5.
About 400 marbles. You simply set up a proportion and solve it as I did in the photo attached.
