75 tickets have been sold so far.
in this question it is clearly mentioned that 45 children's tickets have been sold and this number is 60% of the total tickets sold.
now let's assume the total number of tickets sold be x.
then 60% of x = 45
⇒ x(.6) = 45
by dividing with .6 on both the sides of the equation
⇒ x = 75
therefore 75 tickets have been sold so far.
the final price would be 29.6925
39.59 minus 25% equals 29.6925
you can add the two numbers and divide the result by 2. or you can find the gap between the two numbers and add half of this to the smaller number.