18. Mitch cares only about how much he can write.

Because a pen will write 7 miles of text and a pencil will write 35 miles of text, Mitch considers them perfect 5-to-1 substitutes. If the price of pens is given by P pen and the price of pencils is given by P pencil , and if Mitch’s income is given by Y, use indifference curve analysis to derive the demand curve for pencils.