Answer:
1.47*10^{-8}s
Explanation:
You first calculate the distance traveled by the electron:
[tex]x=vt\\\\x=(0.941(3*10^8m/s))(15.7*10^{-9}s)=4.43m[/tex]
Next, you calculate the relative speed as measure by an observer in the positron, of the electron:
[tex]u'=\frac{u+v}{1+\frac{uv}{c^2}}\\\\u'=\frac{0.941c+0.941c}{1+\frac{(0.941)^2c^2}{c^2}}\\\\u'=0.99c[/tex]
with this relative velocity you calculate the time:
[tex]t=\frac{x}{u'}\\\\t=\frac{4.43m}{0.99c}=1.47*10^{-8}s[/tex]