At what altitude must you descend to from FL210 within 25NM maintaining a GS of 300 kts?

To determine the altitude to descend to from FL210 (21,000 feet) within 25 nautical miles while maintaining a ground speed (GS) of 300 knots, you can use the standard descent formula which considers the altitude to lose and the descent rate.

First, calculate the total altitude to lose, which is 21,000 feet minus the target altitude (which we're determining). The acceptable rate of descent can be estimated at around 1,000 feet per minute for a normal descent.

When descending from FL210 at 300 knots, it's helpful to convert the ground speed into nautical miles per minute. At 300 knots, this translates to 5 nautical miles per minute (since 1 knot equals 1 nautical mile per hour).

To complete the descent within 25 nautical miles, it would take 5 minutes (25 NM / 5 NM per minute). If you descend at a rate of 1,000 feet per minute, you would lose 5,000 feet during that time.

Starting from 21,000 feet and subtracting 5,000 feet gives you an altitude of 16,000 feet. However, since this range includes a selection of specific altitudes, we must select the next lower standard altitude