To find the original cost of each admission ticket, we can start by calculating the total cost before applying the coupon.
You mentioned that the total bill after applying a $20 coupon is $100. This means that the total cost before the coupon was applied was:
\[ 100 + 20 = 120 \]
Now, there are 4 friends (you and 3 friends) who each bought a ticket. To find the original cost of each admission ticket, we can divide the total cost by the number of tickets:
\[ \text{Original cost per ticket} = \frac{120}{4} = 30 \]
Thus, the original cost for each admission ticket is $30.