Dean,
use the ruler tool (It draws a straight line in any angle you wish) and start in the colum that has the value you know (F.e. time 5 minutes), then make the line go through the second column in the exact point of the value you know (F.e. distance 1.5 nautic miles). In the point where the line you drawed crosses the third column (That with the value you did not know, i.e. speed in our case) you can read the result. Hope that helps
