Price mechanism question (economics)

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There are two goods in the market $X1$ and $X2$ and both of their prices are positive. If an agent's utility function is given as: Q1 = amount of X1 agent buys, Q2 = amount of X2 agent buys, $u = Q1 + |Q2-5|$ Identify the demand of this agent.

Note: agent has to spend his initial endowment.

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Clearly it depend on the prices, say $P1$ and $P2$, and on the agent's endowment, say $m$.

There is no point in demanding $Q2 \gt 5$: utility would be increased reducing $Q2$ to $5$ and spending the saved amount on $X1$.

For small endowments the agent will maximise utility by spending on the good with the lower price. Indeed if $P1 \lt P2$ it will always be better to buy more of $X1$ and none of $X2$. And similarly if $P1 \gt P2$ it will be better to buy more of $X2$ and none of $X1$ until $Q2=5$. So

  • if $P1 \lt P2$, then demand is $Q1=m/P1$ of $X1$ and $Q2=0$ of $X2$
  • if $P1 \gt P2$ and $m/P2 \le 5$, then demand is $Q1=0$ of $X1$ and $Q2=m/P2$ of $X2$
  • if $P1 \gt P2$ and $m/P2 \ge 5$, then demand is $Q1=(m-5\times P2)/P1$ of $X1$ and $Q2=5$ of $X2$

If $P1=P2$ then there are several solutions which are convex combinations of those bullet points.


Added much later

Rereading this in the light of Amit's perceptive comment, I should have said something like:

If $P1 \le P2$ it will always be better to buy more of $X1$ and none of $X2$, and this will also be the case with small endowments and $P1 \gt P2$: in either case you get a utility of $Q1+5$.

But for $P1 \gt P2$ and a large endowment $m$ the opposite is true and you can get a higher utility of $Q2-5$ when $m/P2 - 5 \gt m/P1 +5$, i.e. when $m \gt \frac{10\,P1\, P2}{P1-P2}$. So

  • if $P1 \le P2$ or $m \lt \dfrac{10 \,P1\, P2}{P1-P2}$, then demand is $Q1=m/P1$ of $X1$ and $Q2=0$ of $X2$
  • if $P1 \gt P2$ and $m \lt \dfrac{10 \,P1\, P2}{P1-P2}$, then demand is $Q1=0$ of $X1$ and $Q2=m/P2$ of $X2$
  • if $P1 \gt P2$ and $m = \dfrac{10\,P1\, P2}{P1-P2}$ then either all $X1$ and no $X2$ or no $X1$ and all $X2$ are both optimal