First start by getting an expression to represent the
average:
(a + b + c) / 3
Now replace those variables with other expressions
so everything is in terms of a:
=(a + (a + 3) + (a + 9) )/ 3
=(3a + 12) / 3
=3 (a + 12) / 3
= a + 4
Now set up inequalities to figure out which number
is the median (the middle number):
Because b = a + 3, then b > a
Because c = b + 6, then c > b
Now set up the system: c > b > a
Now you know that b is the median.
Subtract it from the average:
a + 4 - b = answer
Again, use the other expressions so this one is in
terms of b.
(b – 3) + 4 – b = answer
= b – b – 3 + 4
= -3 + 4
= 1
B