5=3×2- 1

14=5×3- 1

55= 14×4- 1

274=55×5- 1

1643=274×6- 1

1 1550= 1643×7- 1

Fill in 1643

Fill in the blanks: 1.

Clockwork = 6 degrees per minute.

Minute hand per minute = 1/2 degrees.

Coincidence time =180 ÷ (6-1/2) = 360/11min.

Fill in the blanks: 2

208=2×2×2×2× 13, so 2 and 13 are prime factors of 208, so the sum of all prime factors of 208 is 2+ 13= 15.