Weight anywhere = (mass) x (gravity there)
The acceleration of gravity on Earth is 9.8 m/s² .
So on Earth . . . 
 785 N = (the person's mass) x (9.8 m/s²)
Divide each side by (9.8 m/s²):
 The person's mass = (785N) / (9.8 m/s²)
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On Mars, 
 (his weight) = (his mass) x (acceleration of gravity on Mars)
 298 newtons = (785/9.8 kg) x (acceleration of G on Mars)
Divide each side by (785/9.8 kg):
 Acceleration of gravity on Mars = 
 (298 newton) / (785/9.8 kg) = 3.72 N / kg .