The letters represent a different base ten digit from 0 to 9. None of the numbers can denote a leading zero.
Given that:
O+CTO = BER, and:
OCTOBER is divisible by both 3 and 7.
What is the value of OCTOBER?
An integer is divisible by 3 if and only if the sum of its digits is divisible by 3.
Therefore,
OCTOBER is divisible by 3
=> O+CTO+BER is divisible by 3
=> BER + BER is divisible by 3
=> BER is divisible by 3
Then O + CTO is divisible by 3
=> O+CT+O is divisible by 3
=> O = CT (mod 3)