Use a model of 25 units. 15 units represent chicken pies, 10 units represent beef and mutton pies. Of the 10 units, 7 out of 10 is beef. So 15-7 is 8, therefore 56 beef pies represent 8 units, each unit is 7 pies.
So there are 7 X 7 or 49 beef pies in the beginning.
Then solve the second part. Good luck! Don't give up.