Refer to the current Seattle sectional excerpt around Whidbey Island. One ring of Whidbey Island Class C airspace is charted as "40" over "13." What is the floor of Class C airspace at that ring?

Why: Every current FAA sectional chart uses the same standard altitude-shelf format, shown in that chart's own "Examples of Class B/C Altitudes" legend box: the top number is the ceiling and the bottom number is the floor, both in hundreds of feet MSL. Reading "40" over "13" the same way, that's a ceiling of 4,000 feet MSL and a floor of 1,300 feet MSL. A different ring on this same excerpt, charted "40" over "SFC," has the same 4,000-foot ceiling but extends down to the surface instead.

• 4,000 is this shelf's ceiling (the top number, "40"), not its floor.
• Class C shelf altitudes are referenced to mean sea level, not AGL — the same convention used throughout this chart's own altitude legends.