40 nm per hour = 40 x 6,080 ft per hour = (40 x 6,080) ÷ 60 ft per min
= (40 x 6,080)÷(60 x 60) ft per sec = (40 x 6,080)÷(60 x 60 x 3.28) m per sec
= 20.6 m per sec.
OR
just remember that 1 m/sec = 2 kts (approximately)