PLEASE HELP WITH MATH, ITS EASYYY

Need on this one asap im almost done with this subject

The half-life of a certain material is 0.004 second. approximately how much of a 20-gram sample will be left after 0.016 second? a. 5 grams b. 16 grams c. 0.32 gram d. 1.25 grams

Solve this problem by using a 5d process or writing and solving an equation. no matter which method you use, be sure to define your variable and write an equation to represent the relationship a rectangle has a perimeter of 30 inches. its length is one less than three times its width what are the length and wodyh of the rectangle?