Answer: Every 12th bag has both a T-shirt and socks.
Explanation:
Given: T shirt: Every fourth bag
Socks: Every third bag
Least common multiple of 4 and 3 = 12 as 4 and 3 are coprime (i.e. their highest common factor is 1) and 4 x 3 = 12
It implies every 12th bag has both a T-shirt and socks.
Hence, Every twelfth bag has both a T-shirt and socks.