How do you convert a decimal

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?

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

Find the measure of angle b in the following triangle