He will do both in 12 days since the least common multiple of 3 and 4 is 12. This can be found by making a list of both numbers' multiples. You will see that the first common number or multiple is 12.
Multiples of 3: 3, 6, 9, 12, 15
Multiples of 4: 4, 8, 12, 16, 20
Hope this helped
