HELP ASAP Element X is a radioactive isotope such that every 20 years, its mass decreases by half. Given that the initial mass of a sample of Element X is 770 grams, how long would it be until the mass of the sample reached 400 grams, to the nearest tenth of a year?

Which equation and solution can be used to solve this problem? thirteen less than a number is sixteen. 13-n=16: add 13 to both sides. the answer is 29. n+13=16: subtract 13 from both sides. the answer is 3. n+16=13 subtract 13 from both sides. the answer is 3. n-13=16: add 13 to both sides. the answer is 29.

9/10 divide by -3/15 as a mixed number

Is the product of two rational numbers irrational or rational? first, make a hypothesis by multiplying two rational numbers. then, use variables such as x=a/b and y=c/d and the closure property of integers to prove your hypothesis.