Question

The average age if30 students is 9 years if the age of their teacher is included it become 10 years the age of teachers in year is

Answer 1

40?
Take x1, x2...x30 As the age of students. So
(x1+x2+...)÷30=9
(x1+x2+...)=9×30=270
(x1+x2+...)y÷31=10
270+y÷31=10
270+y=31×10=310
y=310-270=40