For the past n days, the average (arithmetic mean) daily production at a company was 50 units. If today's production of 90 units raises the average to 55 units per day, what is the value of n?

Question: For the past n days, the average (arithmetic mean) daily production at a company was 50 units. If today's production of 90 units raises the average to 55 units per day, what is the value of n?

Solution:
total production in n days is = 50n

ATQ,
50n + 90 = (n + 1)55
or, 50n + 90 = 55n + 55
or, 55n - 50n = 90 - 55
or, 5n = 35
∴ n = 7