Question 980099
To calculate a mean of a set of numbers, we need to add all of the numbers together and divide that result by the total number of numbers we have. 


In our word problem, we are told that the student spent 70, 25, 60, and 30 for the first four months and we are asked to find out how much he can spend in the fifth month so that the mean is 50.  Since we do not know how much he spent in the fifth month, we will represent the fifth month with the letter x.  Our equation will look like the following:


{{{(70+25+60+30+x)/5=50}}}


Simplify the left hand side of our equation, which will give us:


{{{(185+x)/5=50}}}


Next, multiply both sides of our equation by 5 to get rid of the fraction on the left side of the equation:


{{{185+x = 50*5}}}----->{{{185+x=225}}}


Finally, subtract 185 from both sides to obtain the amount he can spend in December (the fifth month):


{{{185-185+x=225-185}}}----->{{{x=40}}}


Therefore, in December, he can spend $40