He also would have known the final decimal digit of powers of 2 (starting at 2^1) is 2, 4, 8, 6, 2, 4, 8, 6, 2, 4, 8, 6…

So, 2^42 ends with the same digit as 2^38, 2^34, …, 2^2, so its last digit is a 4.