The average age of 25 subordinates in an office is 30 years. If the age of Manager is added, the average age increases to 31 years. What is the age of the Manager?

Average age = $$\frac{Total_{age}}{Total _{employee}}$$
or 30 = $$\frac{Total_{age}}{25}$$
hence $$Total_{age}$$ = $$25 \times 30$$ = 750
now adding one manager , total employee will be 26 and New Total Age will be Previous total age + Manager's age
Hence  New average age = $$\frac{Newtotal_{age}}{Total _{employee}+Manger}$$
or 31= $$\frac{Manager_{age}+750}{26}$$
After solving we will get manager's age as 56.