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This post was updated on .
case 1: when they pay their own bill,
utility function is cobb douglas
so x1=M/2px=x2, y1=M/2py=y2
CASE 2: when they divide the bill,
budget equation for 1,
py * y1 + px(x1+x2)/2 =M
U=x1*y1
substitute y1 in U
differentiate wrt x1 and equate to 0
same thing do for x2
we will find x1=x2=2M/3px
y1=y2=M/3py
Since both consumers think that only half of the load of anything that they order (that they wouldn't order when they were eating alone) would fall on them, they wastefully end up spending too much on food and end up with a lower utility.
Hence consumption of x for both decreases.
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