1.2x10^4 s
To determine how long the radio signal takes to travel the distance, just divide the distance by the speed of light. So
4.0x10^8 km / 3.00x10^5 km/s =
Multiply the mantissa, so
4.0 * 3.00 = 12.000
Subtract the exponents
8 - 5 = 3
So the result is 12.000 x 10^3. But since 12 is greater than or equal to 10, we need to divide by 10 and add 1 to the exponent, getting
1.2000 x 10^4 s
And since the least precise value we have has only 2 significant figures, we need to round the result to 2 significant figures, getting
1.2x10^4
As for the units, we had km divided by km/s so
km / km/s = km * s/km = (km s)/km = s/1 = s
Therefore the final result is:
1.2x10^4 s
