Our own ship's speed is 20 kts which means our ship will travel 1/10 that distance in 1/10 of an hour.

The Variable Range Marker (VRM) is set at 2 miles (1/10 of our ships speed).

In 6 minues (1/10 of an hour), target A's echo trail moved quite a bit more than 2 miles on a heading 180° from our ship's heading.

If target A's Relative Motion shown by the echo trail is 4 miles long, that would mean the Relative Speed is 40 kts. Subtracting our own ships true speed of 20 kts tells us target A's True Speed is also 20 kts.

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