We know that
• The total is $90.
,
• Each pen costs $0.75.
,
• Each pencil costs $0.15.
,
• x represents pens, and y represents pencils.
Based on the given information, the expression is

The greatest number of pencils would be 100.
The greatest number of pens would be 100.
Since these values satisfy the equation

Therefore, they can buy 100 pens and 100 pencils.