At a certain restaurant, the average (arithmetic mean) number of customers served for the past x days was 80. If the restaurant serves 120 customers today, raising the average to 90 customers per day, what is the value of x?  Choose the most appropriate option:
A) 2  B) 3  C) 9  D) 15  E) 30
WITHOUTusing the formula, we can see that today the restaurant served 30 customers above the average. The total amount ABOVE the average must equal total amount BELOW the average. This additional 30 customers must offset the “deficit” below the average of 90 created on the x days the restaurant served only 80 customers per day.   30/10= 
<gadget id="calculator">30/10</gadget>
<output>3</output>
3 days.Choice (A).   WITHthe formula, we can set up the following:   90= (80x + 120)/(x + 1)   90x + 90 = 80x + 120   10x = 30   x = 3Answer Choice (3)
<result>B</result>
At a certain restaurant, the average (arithmetic mean) number of customers served for the past x days was 80. If the restaurant serves 120 customers today, raising the average to 90 customers per day, what is the value of x?