marginal utility of consuming vs marginal rate of substitution

Question

good day all

I am taking second year economics, but my textbook is not very clear on this topic

A quoted from

Intermediate Microeconomics and Its Application. Nicholson, Snyder. 11th Edition.

Used in question: 2 Extracts from textbook, 1 text and 1 graph:

Indifference Curve Used as reference:

Text Extract

Tutorial Question given by university:

My question:

From the extract, I understand that the MRS is determined by the change in hamburgers / change in softdrinks which gives MRS = |2|.

From my tutorial question, slightly different terminology was used ("consumption"), which resulted the answer shown.

I do not understand why the answer for the tutorial question is 1/5 and not 5 (based on the example from the book).

Help would be very much appreciated!

Answer 1

This is because the question is asking the MRS of soda for hotdogs not the MRS of hotdogs for soda.

i.e

$MRS(MU(soda),MU(hotdogs))=MU(soda)/MU(hamburgers)= 2/10=1/5$

To get 5 you need to solve the MRS of hotdogs to soda

$MRS(MU(hotdogs),MU(soda))=MU(hamburgers)/MU(Soda)=10/2=5$

Hope this helps.