Since he only makes commission on sales over $1000, he makes commission on $1500 (we find this by doing 2500-1000.) His commission is 15%, so find 15% of $1500:
[tex].15*1500=225[/tex]
So we know how makes $225 in commission.
Now we need to add this to his fixed pay to get his total pay:
[tex]225+300=525[/tex]
So his total pay is $525, which is answer 'a'
Hope I helped, and let me know if you have any questions :)