Please help me out with this question please

Dylan and dusty plan to take weekly surfing lessons together. if the 2-hour lessons are $20 per person and they plan to spend $100 each on new surfboards, what is the maximum number of lessons the two can take if the total amount spent for lessons and surfboards is at most $480?

Determine the number of girls in a math class with 87 boys if there are 25 girls for every 75 boys

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?