I need some help please <3 I don't know how to find the volume

If i have 20$ and i gave 5$ away how much i got

The boiling point of water at an elevation of 0 feet is 212 degrees fahrenheit (°f). for every 1,000 feet of increase in elevation, the boiling point of water decreases by about 2°f. which of the following represents this relationship if b is the boiling point of water at an elevation of e thousand feet? a) e = 2b - 212 b) b = 2e - 212 c) e = -2b + 212 d) b = -2e + 212

If the tip varies directly with the number of guest which equation represents between the tip,t, and the number of guest,g?