Answer:
i = 61 degree
Explanation:
Given,

Now, by the snell's law

Now,
Sin i / sin r = n 2 / n 1
sin i / sin r (45 - 24.09) = 2.45 / 1
i = 60.97 degree
First speed = 20km/h
Time = 3 hours
Distance = 3×20
<h3> = <u>60 km</u></h3>
Second speed = 30km/h
Time = 4 hours
Distance = 4×30
<h3> = <u>120 km</u></h3>
Total distance = 60+120 = <u>180km</u>
Total time = 3+4 =<u> 7 hours</u>
Average speed = 180/7
<h3> = <u>25.71</u><u> </u><u>km</u><u>/</u><u>h</u></h3>
Hope this will help...
Sound is a longitudinal wave.
-- The source of most of the energy that radiates from the sun is nuclear energy.
-- Most of the energy that radiates from the sun is electromagnetic energy.
-- Heat energy is part of the electromagnetic energy that radiates from the sun.
Other parts include radio, microwave, visible light, ultraviolet, and X-ray energy.