Question 160
Initially, a blackboard has written on it the numbers $11$ and $13$. Each minute an extra number appears on the blackboard, equaling the sum of two numbers already written on the blackboard. Prove that:
a) the number $86$ cannot appear on the blackboard;
b) it is possible for $2015$ to appear on the blackboard at some point.
a) the number $86$ cannot appear on the blackboard;
b) it is possible for $2015$ to appear on the blackboard at some point.