Answer:
b.  (√15)/4
Step-by-step explanation:
Since Sin Ф = (opposite side)/Hypotenuse, we have 2 sides of a right triangle.  
Use Pythagorean theorem to solve for the missing leg (the adjacent side)
  1² + b² = 4²
     1 + b² = 16
             b² = 15
                  b = √15
So the adjacent side is √15, so Cos Ф = (√15)/4
 
        
                    
             
        
        
        
Minus the 7 
-2x = -6
Divide by -2 
X = 3
        
                    
             
        
        
        
The answer is: 520
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The decay equation is of the form

where
N₀ = initial amount
k = decay constant
t = time
The half-life is 1.03 x 10⁵ years. Therefore

Because N₀ =  5 x 10⁹ atoms, the number of atoms remaining when t  = 4.12 x 10⁵ years is

Answer: 3.125 x 10⁸ atoms