The total of tickets sold would be 100% or 1, so we have 1/6 of total tickets for kids and 1/2 of total of tickets for adults, for finding the part of tickets sold for seniors we calculate: 1/6 + 1/2 + x = 1 here x is the part of tickets for seniors.
Solve for x: x=1/3.
Now let total of tickets be "T", we know the amount of tickets sold for seniors (40) so we can say: 1/3*T = 40, solve for T and get T = 40*3 = 120 tickets sold.
Finally we can calculate the money collected from tickets sold multiplying the cost*the part of tickets sold for each group*120 and add all of them, this is:
$2*1/6*120 + $6*1/2*120 + $3*1/3*120 = $40 + $360 + $120 = $520
The money collected from ticket sales is: $520
