1. Percent error is 
The guess is 15.5mi, and the actual distance is 14.5 mi.
Percent error is
2. The cost will be $31.95*(1+0.075) = $34.35
The 1 represents 100% of the original amount of $31.95, and now we add 7.5% (0.075) to the 100%. In other words we multiply $31.95 by 107.5%.
3. His commission will be $165 out of $4125, or $165/$4215 = 0.039 = 3.9%
4. The principal (amount of money you start with) is $12000, and the interest rate is 15% of that.
15% of $1800 = 0.15*$12000 = $1800
The guess is 15.5mi, and the actual distance is 14.5 mi.
Percent error is
2. The cost will be $31.95*(1+0.075) = $34.35
The 1 represents 100% of the original amount of $31.95, and now we add 7.5% (0.075) to the 100%. In other words we multiply $31.95 by 107.5%.
3. His commission will be $165 out of $4125, or $165/$4215 = 0.039 = 3.9%
4. The principal (amount of money you start with) is $12000, and the interest rate is 15% of that.
15% of $1800 = 0.15*$12000 = $1800