X - the number of pencils
y - the number of pens
She has twice as many pencils as she has pens.
Tom borrows a pen, so she has y-1 pens. Now she has ten less pens than pencils.
Set 2y and y+9 equal to each other:
She has 9 pens.
y - the number of pens
She has twice as many pencils as she has pens.
Tom borrows a pen, so she has y-1 pens. Now she has ten less pens than pencils.
Set 2y and y+9 equal to each other:
She has 9 pens.