At a certain restaurant, the average (arithmetic mean) number of customers served for the past x days was 60. If the restaurant serves 120 customers today, raising the average to 70 customers per day, what is the value of x?
Select the correct option
A) 2  B) 5  C) 9  D) 15  E) 30
WITHOUTusing the formula, we can see that today the restaurant served 50 customers above the average. The total amount ABOVE the average must equal total amount BELOW the average. This additional 50 customers must offset the “deficit” below the average of 70 created on the x days the restaurant served only 60 customers per day.   50/10= 
<gadget id="calculator">50/10</gadget>
<output>5</output>
5 days.Choice (A).   WITHthe formula, we can set up the following:   70= (60x + 120)/(x + 1)   70x + 70 = 60x + 120   10x = 50   x = 5Answer Choice (5)
<result>B</result>
At a certain restaurant, the average (arithmetic mean) number of customers served for the past x days was 60. If the restaurant serves 120 customers today, raising the average to 70 customers per day, what is the value of x?