It's really helpful to know that 1/7 = 0.142857142857... repeating.

Do you need to memorize this? I would.

It's incredibly useful to know 1/2 through 1/12 as decimals. Thank me later.

In our case, the 1/7 decimal repeats every 6 digits. That means every 6th digit, or every multiple of 6, is a 7. That means the 48th, which is a multiple of 6, is a 7. Then we know that 1 and 4 come immediately after the 7, so the 49th is 1 and the 50th is 4.

The answer is 4.

Thanks Carcass, I misread the question and mistakenly calculated 15th digit