Assumption method :


Assume all 368 durians Kai sold were small durians.


Amount collected would be
= 368 x $5
= $1840


Price difference between a big durian and a small durian

= $9 - $5
= $4

So if Kai sells 1 big durian instead of 1 small durian , he earns $4 more.

$2760 - $1840 = $920

This means that he has earned $920 more than he would have if all the 368 durians he sold were small durians.

Since selling 1 big durian earns him $4 more than selling 1 small durian,

$920 ÷ $4 = 230

Selling 230 big durians earned him $920 more than selling 230 small durians.

Therefore , he sold 230 large durians.