Via half-life equation we have:

Where the initial amount is 50 grams, half-life is 4 minutes, and time elapsed is 12 minutes. By plugging those values in we get:

There is 6.25 grams left of Ra-229 after 12 minutes.
Explanation:
formula for energy is k. e = ½mv²
m= 9
v= 75
k. e = ½×9×75 =337•5
Haven't taken physics but I would assume if her friend is standing in front of her that you would add up the speeds and get 30 km/hr.
We use only one variable at a time to find the accurate result. We want to see how the result of experiment changes everytime with a single variable.