Will you be my friend pls

Tom wants to order tickets online so that he and three of his friends can go to a water park the cost of the tickets is 16.00 per person there is also a 2.50 one-time service fee for ordering tickets online write an expression in term of n that represents the cost for n ordering tickets online

Find the value of x. also give me an explanation so i can complete it to other questions as well, : )

The population of a city has increased by 35% since it was last measured. if the current population is 86,400 what was the previous population