What would a person's weight be on Mars compared to one's weight on Earth?
The strength of gravity is proportional to M/R^2 where M and R are the mass and radius of the planet.
The weight ratio for planets 1 and 2 is
W1/W2 = (M1/M2)(R2/R1)^2
For specific value for Mars, see
http://www.universetoday.com/22603/mars-compared-to-earth/