Answer:
1.526 m
Explanation:
The motion sensor sends sound waves in air which travels at a speed of 343 m per second at 20 degree. If d be the distance of the reflecting object
total distance travelled = 2d
Time taken t = 8.9 x 10⁻³ s
velocity v = 2d / t
343 = 2 d / (8.9 x 10⁻³)
d = 1.526 m