A standby power unit ran on 4 days of the first week, turning out an average of 55 units of electricity a day, and on 6 days of the second week at an average of 45 units a day. What was its average daily output over the 10 days it ran?

The two weeks gave $4\times 55=220$ units and $6\times 45=270$ units, a total of 490 units over 10 running days: 490/10 = 49 units a day. 50 is the plain midpoint of 55 and 45, which would only hold if the unit had run the same number of days in each week; it ran longer in the weaker week, so the average must fall below 50. 51 attaches the day counts to the wrong weeks. 490 is the total output, not the daily average.