Petrolyn motor oil is a combination of natural oil and synthetic oil. it contains 3 liters of natural oil for every 2 liters of synthetic oil. in order to make 345 liters of petrolyn oil, how many liters of natural oil are needed

Ratio of natural oil to synthetic oil in Petrolyn oil is 3:2. (notice how the order matters, if I wrote 2:3 then I would be writing the ratio of synthetic to natural)

How I think about it is first add up the "numbers in the ratio", which in this case 3+2.
Then take the quantity and divide it by the sum of the "numbers in the ratio". The quantity in this case is 345 and the sum of the "numbers in the ratio" is 5, so 345 / 5 which is 69. Now we can see the ratio actually is 3(69) : 2(69), so when we multiply 3 by 69 we get 207.
This means there are 207 liters of natural oil in 345 liters of petrolyn oil.

Now there is an easier way to do it:
We know ratio of natural to synthetic is 3:2. 
To write the amount of natural oil in Petrolyn oil in fraction form is: 3/5 (we got 5 from 3+2 which is the sum of the "numbers in the ratio" as I call it)

To find out how much natural oil is in 345 liters of Petrolyn oil we simply multiply 345 by 3/5(the fraction of natural oil).

345 x 3/5 = 207 liters which is the same answer we got using the long way!

Hope you understand how to solve these types of problems now,
and May the Force Be With You!

-Jabba