in an ordinary day with no discount if u see the parretn you can get the cost per ticket by dividing the cost by number of tickets
1st : 21/3 =7
2nd: 35/5=7
3rd: 56/8 = 7
so the ticket in an ordinary day = $7
in friday they make discount by $2 so the new value of the ticket = the old value -2
so the ticket on friday = $5
so if you want 9 tickets
9*5 = $45