A50.0 mg sample of an unknown radioactive substance was placed in storage and its mass measured periodically. after 19.7 days, the amount of radioactive substance had decreased to 3.13 mg. what is the half-life of the unknown radioactive substance?

An astronaut in space cannot use a scale or balance to weigh objects because there is no gravity. but she does have devices to measure distance and time accurately. she knows her own mass is 77.4 kg , but she is unsure of the mass of a large gas canister in the airless rocket. when this canister is approaching her at 3.50 m/s , she pushes against it, which slows it down to 1.30 m/s (but does not reverse it) and gives her a speed of 2.60 m/s . what is the mass of the canister?