The average of $6$ observations is $12$. $A$ new seventh observation is included and the new average is decreased by $1$. The seventh observation is

$A$ library has an average of $510$ visitors on Sundays and $240$ on other days. The average number of visitors per day in a month of $30$ days beginning with a Sunday is:

The average of the two-digit numbers,which remain the same when the digits interchange their position,is:

The batting average for $40$ innings of a cricketer is $50$ runs. His highest score exceeds his lowest score by $172$ runs. If these two innings are excluded,the average of the remaining $38$ innings is $48$ runs. The highest score of the player is

The average temperature of Monday,Tuesday,Wednesday and Thursday is $60^{\circ}$. The average temperature for Tuesday,Wednesday,Thursday and Friday is $63^{\circ}$. If the ratio of temperature for Monday and Friday is $21: 25,$ then what is the temperature of Friday? (in $^{\circ}$)

The average age of $8$ members of a group is $40 \text{ years}$. The age of $1$ member is $55 \text{ years}$. After his retirement,a new member of age $39 \text{ years}$ joins the group. What is the effect of this replacement on the average age of the group?