To find the original cost for each admission ticket, we start with the total bill before applying the coupon.
The total bill is $100, and the coupon saves $20. Therefore, the total cost of the admission tickets before the coupon is applied is:
\[ 100 + 20 = 120 \]
This $120 is the total cost for all four admission tickets. To find the cost of each individual ticket, we divide the total cost by the number of tickets:
\[ \text{Cost per ticket} = \frac{120}{4} = 30 \]
So, the original cost for each admission ticket is $30.