Nicholas bought 3 erasers.
To do this, you first subtract $4 from $13 to get how much money he has left to buy the erasers.
$13-$4=9
After this, you divide $9 by $3 to find how many erasers he bought. $9 is how much money he has left while $3 is the cost of each eraser.
$9 / $3 = 3
Therefore, Nicholas bought 3 erasers.
I hope this helped you!