marginal utility of consuming vs marginal rate of substitution
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.