Answer:
Neither "matter" nor "energy" can be created or destroyed, but they can be changed from one form to the other.
Explanation:
The answer is 68 F. i hope this helps
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%
-- Electric field lines DO never cross. <em>(A)
</em>
-- Electric field lines that are close together DO indicate a stronger electric field. <em>(B)
</em>
-- Electric field lines DO not affect the charge that created them. <em>(C)</em>
-- Electric field lines DON'T begin on north poles and end on south poles. North and South "poles" are the way we talk about magnets, not electric charges.
