The space probe Pioneer 11 was launched April 5, 1973, and reached Jupiter in December 1974, traveling a distance of 998 million km. How long did it take an electromagnetic signal to travel to Earth from Pioneer 11 when it was near Jupiter?
This image is in the public domain: http://www.jsc.nasa.gov/policies.html#Guidelines
Solution:
To do this question, we must know that electromagnetic signals travel at the same speed as light.
The speed of light = 2.998\times 10^8 \frac{m}{s}.
Knowing that, we simply have to divide the distance it took for the signal to travel by the speed of electromagnetic signals.
\dfrac{998\times 10^9\text{m}}{2.998\times 10^8\frac{\text{m}}{\text{s}}}\,=\,3.33\times 10^3 seconds