One ticket costs $24.50 - $19 more for evening
Six tickets cost 6 ( $24.50 - $19 ) more
Answer: 6 ( $24.50 - $19 )
Problem 18 is D by the way
To set the equation,we can first find how much price would be raised within a year:
=500×(1+7%)
=500×1.07
As the years go by the increase would accumulate,and the equation would be:

Hope it helps!
#13 would be 100%.
#15 would be $9.90.
#17 would be 408.
#19 would be 27840.
#21 would be 26.
Sorry for the late reply.