From the given information we know that he can get 3 dinars for every $4, then we can express the following proportion:
3dinars /4 dollars
We express it with the dinars in the numerator since we are asked to find how many dinars would he get if he exchanged $300, we can calculate the corresponding dinars by multiplying the proportion by 300, like this:
300*3/4 = 225
Then Mike would get 225 dinars if he exchanges $300