Iam supposed to write 'half of the difference between a and b, where a is bigger than b' as an expression but i don't understand how. can i have an explanation on how and why?

Raymond just got done jumping at super bounce trampoline center. the total cost of his session was $43.25 . he had to pay a $7 entrance fee and $1.25 for every minute he was on the trampoline. write an equation to determine the number of minutes (t) that raymond was on the trampoline.