Question

Mary has twice as many pencils as she has pens. Tom borrows a pen. Now, Mary has ten less pens than pencils. How many pencils does Mary does?

Answer 1

X - the number of pencils
y - the number of pens

She has twice as many pencils as she has pens.
$x=2y$

Tom borrows a pen, so she has y-1 pens. Now she has ten less pens than pencils.
$x-10=y-1 \\ x=y-1+10 \\ x=y+9$

Set 2y and y+9 equal to each other:
$2y=y+9 \\ 2y-y=9 \\ y=9$
She has 9 pens.