A particular star is at a distance of about $$8.1\times 10^{13}\ km$$ from the Earth. Assuring that light travels at $$3\times 108\ m$$ per second, find how long does light take from that star to reach the Earth.
As, $$1\ km=1000\ m$$
Total distance between the earth and the star $$=8.1\times 10^{13}\ km=8.1\times 10^{13}\times 10^{3}\ m=8.1\times10^{16}\ m$$
Speed of light $$=3\times 10^8\ m$$ per second.
Also, we know that , $$\text{distance = speed}\times \text{time}$$
Hence,
$$\text{time that light would take to reach the earth =} \cfrac{\text{distance of star from earth}}{\text{speed of light}}$$
$$=\dfrac{8.1\times 10^{16}}{3\times 10^8}$$
$$=\dfrac{8.1}{3}\times 10^8\cdots\cdots\left[\because \cfrac{a^m}{a^n}=a^{m-n}\right]$$
$$=2.7\times 10^8\ s$$