An amount equaling 28 kilograms of a radioactive substance has a half-life of 10 years. How many years will it take the substance to decay naturally to only 7 kilograms

In a heat engine of 1000 j of heat enters the system and the piston does 500 j of work what is the final internal energy of the system if the inital energy was 2000 j we have to do all of these down here 1)write the equation 2)list out your know variables 3)plug the numbers into the equations 4)solve 5)write your solution statemtn that includes inital energuy and final energuy added