Question

A star is $8.1\times {10}^{13}km$ away from the earth. Suppose right travels at the speed of $3.0\times {10}^{5}km$ per second, how long will it take light from the star to reach the earth?

• A. $7.5\times {10}^{3}hrs$
• B. $7.5\times {10}^{4}hrs$
• C. $2.5\times {10}^{10}sec$
• D. $2.7\times {10}^{11}sec$

Answer 1

Answer: (B)

$7.5\times {10}^{4}hrs$

Time taken $=\frac{Distance}{Speed}=\frac{8.1\times {10}^{13}}{3.0\times {10}^5}seconds$
$=(2.7\times {10}^8\times \frac{1}{60}\times \frac{1}{60})hrs$
$=\frac{2.7\times {10}^6}{3600}hrs=\left(\frac{2.7\times 100\times {10}^4}{36}\right)hrs$
$=(7.5\times {10}^4)hrs$.