It is given that Alexander earns 12% commission each week.
Last week, he sold $5300 worth of stocks.
The amount he made last week in commission was
So, he made $636 last week in commission
He averages the same amount each week.
So, it is convenient to consider that he makes $636 in a week.
In 2011, there were 52 weeks.
So, the amount he made in 2011 is
So, he made $33072 in the year 2011