Answer:
20 cards
Step-by-step explanation:
Given: Your stack is 12 mm tall and is has 48 cards.
Your friends stack is 5mm tall and has c cards.
To Find: How many cards are in your friends stack?
Solution:
Stack which is 12 mm tall has cards = 48
So, stack which is 1 mm tall has cards = 
= 
So, stack which is 5 mm tall has cards = 
= 
Hence there are 20 cards in the stack of your friend .