The average marks obtained by a group of 10 students were 20. One student left the group as a result of which the average of the remaining students rose to 21. But another student joined, as a result of which the average marks of the group dropped a bit and became 20.6. What were the average marks obtained by the student who left and the one who joined?

Average of 10 students = 20
Sum of their marks  $$= 10\times 20 = 200$$

1 Student left now total students=9

Sum of their marks  $$= 9\times 21 = 189$$

Left student marks =200-189=11

Another student Joined now total students=10

 New Average of 10 students = 20.6

Sum of their marks  $$= 10\times 20.6 = 206$$

New  student joined marks =206-189=17

total marks obtained by both student(who left & one joined)

=17+11=28

average$$= \frac{28}{2} = 14$$