$5 per year.
1. Fill in the table
Years 200, Cost $1000
Years 400, Cost $2000
Years 500, Cost $2500
2. c = 5t
3. Table
Years -200, Cost $3000
Years -400, Cost $6000
Years -500, Cost $7500
4. c = -15t
5. For traveling forward, the constant of proportionality is 5, positive.
For traveling backward, the constant of proportionality is -15, negative.
The cost must always be a positive number. The cost is the constant of proportionality times the number of years. Since for traveling forward, the number of years is positive, the constant of proportionality is also positive since positive times positive is positive, and it will give a positive cost.
For traveling backward in time, the number of years is negative. To get a positive cost, the constant of proportionality must also be negative since negative times negative is positive. If the constant of proportionality were positive then the cost would be negative.