1/50
This is just a matter of multiplying the two fractions you've been given together. You can make things easier for you by doing some reduction of the fraction before you multiply the numbers together.
6/17 * 17/300 = (6 * 17)/(17 * 300)
Since you have a factor of 17 both top and bottom, they cancel each other out, giving
(6 * 17)/(17 * 300) = 6/300
And since the greatest common factor for 6 and 300 is 6, you can divide both top and bottom by 6 giving
6/300 = 1/50
So Pluto as 1/50 of the diameter as Saturn.
