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)