How can you use four rods that measure 2 centimeters 5 centimeters 7 centimeters and 9 centimeters to measure a length of 1 centimeters?

Put the 7cm and 5 cm rods together = 12 cm

Put the 9cm and 2cm rods together = 11 cm

Difference between is 1cm thus, you can use that difference to measure 1 cm.