First, we need to make the denominators equal. An easy way to do this is to multiply the 2 denominators together. 7x9= 63. So now the denominators are 1/63 and 8/63, but now we need to fix the numerators. since we multiplied 7 by 9 we should also multiply 1 by 9. This makes the medium shake 9/63 the amount of a large. For the large shake we multiplied 9 by 7 so we should multiply 8 by 7. This makes the large shake. 56/63
Now we have 56/63-8/63 easier seen as 56-8 with a denominator of 63.
After subtracting we can see that a medium shake uses 48/63 pints of milk. But we aren't done yet. Now we need to simplify. I am assuming you can do that.
The answer is 16/21 pints of milk to create a medium shake.
