1) The half-life is the time required for a substance to reduce to half its initial value. In formulas:

(1)
where
m(t) is the amount of substance left at time t
m0 is the initial mass

is the half-life
In this problem, the half-life of the substance is 20 years:

therefore, the fraction of sample left after t=40 years will be

So, only 1/4 of the original sample will be left, which corresponds to 25%.
2) We can use again formula (1), by re-arranging it:

If we use m(t)=10 g (mass of uranium left at time t), and

(the time is equal to 4 half lifes), we get

So, the initial sample of uranium was 160 g.
Answer: B. 23 inches to the left
Explanation: A vector quantity involves both magnitude and direction.
1.Landslide 2. Delta 3. Moving water 4. Erosion 5. Abrasion and Deflation
6. Winds 8. Sediment it can erode
Sorry, don't know 7.
Answer:
684.5 is the weight on mars and 1813 on earth
Explanation:
185*3.7=684.5 185*9.8=1813 you multiply for earth by 9.8 because that's the gravity on earth and you multiply by 3.7 because that's the gravity on mars