To find out how much Stan is paid a day, simply find out how many miles Stan drives per day, and multiply that with the amount Stan is paid per mile (in this case $3.50 per mile). Then, add that with the daily amount Stan earned (which is $75).
For example, on Monday, Stan drove a total of 30 miles (and he's paid $3.50 per mile).
Simply multiply $3.50 (per mile) with 30 (total number of miles driven), and that should equate to $105 earned for driving a total of 30 miles.
Then, add $105 (earned from driving 30 miles) to $75 (daily pay) and that equals $180.
Stan had been paid a total of $180 on Monday.