answer: marginal utility of the fourth brownie is 0.
explanation: marginal utility is the addtional utility gained from the consumption of one more unit of a good. it is given by,
![mu{n} = tu{n} -tu{n-1}](/tex.php?f= mu{n} = tu{n} -tu{n-1} )
so, marginal utility at the 4th unit will be given by
![mu{4} = tu{4} -tu{3}](/tex.php?f= mu{4} = tu{4} -tu{3} )
since, the values are not given here, you can simply find marginal utility at fourth unit by subtracting total utility at 3rd unit from total utility at 4th unit.