Please help thank you ahead of time

Okay so basically, i’m pretty sure you have to make them catch up with each other, if i’m correct), so if it’s 30 to each student plus 2500 then 2 students is 60 bucks, 3 students is 90 and so on so forththen if 1 student - 60 then 2 is 120 and vice versa ya know. So keep adding on the amount due for each student plus the extra amount which doesn’t change and make them catch up with each other. Keep on adding student after student and adding the fee until they match up. Sorry it’s not the exact answer but i hope this explains it... if i’m right