If the total bill to get into the carnival was $100 for you and your three friends (which means a total of 4 people), and you have a coupon for $20 off, the effective total cost to be paid is:
\[ 100 - 20 = 80 \]
Now, to find the original cost of each admission ticket, we divide the original total cost by the number of people:
\[ \text{Original cost per ticket} = \frac{100}{4} = 25 \]
So the original cost for each admission ticket is $25.