The total quantity of meatloaf that Mike uses for his recipe = 7 pounds
The ratio of beef to pork that needs to be used = 3:1
Let us assume the common ratio to be = x
Then
3x + x = 7
4x = 7
x = 7/4 
So
The amount of pork that is 
needed for the recipe of Mike = 1 * (7/4) pounds
 = (7/4) pounds
 = 1 3/4 pounds
So the quantity of pork that is required to make the recipe of Mike is 1 3/4 pounds. I hope the procedure is simple enough for you to understand.