when the function of f(x) varies directly with X and f(x) equals to 51X equals 50 what is f(x) when X equals five

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?

What is not true about kl and mn? what is mn? (show )

Astudent showed the steps below while solving the inequality |x-4|< 1 by graphing.