The acceleration due to gravity of the planet X is 1 m/s².
The given parameters;
- height above the ground, h = 100 m
- initial velocity of the rock, u = 15 m/s
- time of motion of the rock, t = 10 s
The acceleration due to gravity is calculated as follows;

Thus, the acceleration due to gravity of the planet X is 1 m/s²
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We know average speed =total distance/time taken
So avg speed=(85+63)/(5+5)=14.8km/hr