Terms of $$3:2$$ and $$3$$ and $$2$$
Sum of these terms$$=3+2=5$$
Sheela will get $$3/5$$ of total pens and Sangeeta will get $$2/5$$ of total pens.
Number of pens with Sheela$$=\dfrac{3}{5}\times 20=12$$
Number of pens with Sangeeta$$=\dfrac{2}{5}\times 20=8$$.