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:	A) 2	B) 5	C) 9	D) 15	E) 30

Rationale: "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 = 5 days . choice ( a ) . withthe formula , we can set up the following : 70 = ( 60 x + 120 ) / ( x + 1 ) 70 x + 70 = 60 x + 120 10 x = 50 x = 5 answer choice ( b )"
Answer: B