Question 977912
Hi there,
If the mean of 6 numbers is 10
then the total was 6 x 10 = 60
When another number is added
then this totals 7 numbers
If the new mean is 9 then the new
total = 63
New total - Old total
63 - 60 = 3
Therefore 3 must have been 
the number added.
Hope this helps:-)