This is how I answered the question. If you let the siblings be represented by the letters a,b,c,d,e,f (there are six of them including Anita), then you can set up the following equation:
(a+b+c+d+e+f)/6 = 6.
This comes from the formula for a mean.
In ten years, each sibling will have added 10 years to their age, so this is the equation for the mean of their ages ten years from now:
By adding all of the 10's in the numerator we get:
We can seperate this fraction into:
60/6 + (a+b+c+d+e+f)/6
60/6 = 10 and we know that (a+b+c+d+e+f)/6 = 6, so the answer is:
10+6 = 16.
So in ten years the mean age increased 10 years.