Advanced physics

the speed of a sound Air at 0 degrees Celsius is 331m/s

at 20 degrees Celsius is 343m/s

so I want to know why 340m/s is used on this question?

QUESTION 3 3. Sipho and his friend Kadesh decide to do an investigation on the speed of sound. Sipho holds a drum 10 m from a wall while Kadesh stands some distance away holding a sound detector in his hand. When Sipho hits the drum ONCE the sound detector receives TWO sounds. Kadesh Sipho wall 10 m 3.1 Explain why the detector receives two sounds. 3.2 The second sound reached the detector 0,15 s after Sipho hits the drum. Calculate the distance between Sipho and Kadesh. 3.3 What will be the result if Sipho hits the drum in a vacuum? Give a reason for your answer. [8]