The two different isotopes have weights :
w1 = 78.918 amu
w2 = 80.916 amu
average weight w3 = 79.903 amu
The mixing of two components can be modeled as
let the fraction of w1 be 'x'
hence 
now this is a linear equation in 'x'. Substituting the values we get
x = 0.507
hence the percentage of Br79 = 50.7% and the percentage of BR81 = 49.3%
Answer:
The angular acceleration of the wheel is -6.54 rad/s²
Explanation:
We'll use the equations of motion for this.
w = 2πf
f = 75 rpm = 1.25 rps = 1.25 rev/s
w₀ = initial angular velocity = 2π × 1.25 = 7.85 rad/s
w = final angular velocity = 0 rad/s
t = 1.2 s
α = ?
w = w₀ + αt
0 = 7.85 + 1.2α
α = 7.85/1.2 = - 6.54 rad/s²
I feel like it could be A
Option 2 is your answer :)
