DAY 67

Comment

You could look for specific number-triples whose average is one of the numbers, to see whether a pattern emerges for the average of the other two numbers.
A more general, but also more secure and direct approach, is to find the value of a + b + c, then the value of b + c, then the average of b + c.

Answer

The average of b and c is also a.

Readers' methods

Alisha C, Kerry:
Say a is 5, then the average would have to be 5.
So b and c could be 2 and 8, say, in which case their average would 5, which is the value I chose for a.
I think this would always work.

Carole S, Urmston M41:
(a + b + c) / 3 = a,
so a + b + c = 3a,
so b + c = 2a.
The average of b and c is
(b + c) / 2 = 2a / 2 = a.