The total you want to make is $1.50, which is a multiple of 10 cents.
Each dime is 10 cents; but each quarter is 25 cents.
To use quarters to get a total that is a multiple of 10 cents, you need an even number of quarters.
So you can make the total of $1.50 any time the number of quarters is even.
You can of course use 0 quarters and make the whole $1.50 using dimes.
And $1.50 is 6 quarters, so 6 is the largest number of quarters you can use to make a total of $1.50.
So the number of quarters you can use to make $1.50 is any even number from 0 to 6 -- i.e., 0, 2, 4, or 6.
So there are four different ways Mia can pay for the $1.50 drink with quarters and dimes.