1. Here's the beginning of some working, hopefully I've interpreted the question correctly! :P

Width of the path = x

Can you take it from there? (Remember, width must be positive)

2. t = time take and s = average speed of the aircraft

Can you take it from there? (Remember, speed, a scalar quantity, must be positive)

3. where x = number of seniors and c = cost per person

Can you take it from there? (Remember, x must be a positive number)