Source: p 109, Question 5.9, Principles of Microeconomics, 7 Ed, 2014, by N Gregory Mankiw
= Question 5.7, Principles of Microeconomics, 4 Ed, 2008, by N Gregory Mankiw
9. $\color{green } { \text { Before looking at the price,} }$ Jessie spends $d$ dollars on something (abbreviate this S). What is Jessie’s price elasticity of demand?
Given Answer: Jessie's price elasticity of demand is one, because he spends the same amount on S, no matter what the price, which means $\color{darkred} { \text { his percentage change in quantity } }$ is equal to the percentage change in price.
From p 91: Price elasticity of demand $= \dfrac{ \text{ Percentage change in quantity demanded }} { \text{ Percentage change in price } } $
The green implies Jessie's ignorance of prices, so % change in price = 0. Yet please explain the answer? Especially the red, because the question implies nothing about the % change in quantity demanded?
Footnote: I slightly generalised the question. Mankiw writes 'gas' instead of 'something'.