At a certain restaurant, the average (arithmetic mean) number of customers served for the past x days was 85. If the restaurant serves 120 customers today, raising the average to 90 customers per day, what is the value of x?
Choose one:
A) 2
B) 5
C) 6
D) 15
E) 30

Rationale: "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 85 customers per day . 30 / 5 = 6 days . choice ( a ) . withthe formula , we can set up the following : 90 = ( 85 x + 120 ) / ( x + 1 ) 90 x + 90 = 85 x + 120 5 x = 30 x = 6 answer choice ( c )"
Answer: C