Answer: 590
Ok, lets find out how much he earn for every 90 pens he sells.
90x$2=$180 plus bonus $20 equals $200
So, we will now find what are the maximum sets of $200 in $1300.
$1300 divide by $200 equals 6 with remainder 100 and lets not care about the remainder first.
For the salesman to earn 6x$200 (which is $1200) ,he must sell 90x6 pens which is 540 pens.
What about the remaining $100? Well, he just need to sell 50 more so, 50 plus 540 = 590.
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