The answer is; 15 years
It takes 687 days for mars to orbit the sun once. 8 mars years would, therefore, mean;
8 * 687 = 5496 days
The earth takes 365 days to orbit the sun once. Therefore, the earth will orbit the following number of times in 8 Mar years;
5496/365 = 15.058
Which is approximately 15 years
