A boy walks to his school at a distance of 6𝑘𝑚 with constant speed of 2.5 𝑘𝑚/ℎ𝑜𝑢𝑟 and walks back with a constant speed of 4 𝑘𝑚/ℎ𝑟. His average speed for round trip expressed in 𝑘𝑚/ℎ𝑜𝑢𝑟, is

Question : A boy walks to his school at a distance of 6𝑘𝑚 with constant speed of 2.5 𝑘𝑚/ℎ𝑜𝑢𝑟 and walks back with a constant speed of 4 𝑘𝑚/ℎ𝑟. His average speed for round trip expressed in 𝑘𝑚/ℎ𝑜𝑢𝑟, is