The cross section of seven circular metal metal rods bound together by a strap is shown. if the the diameter of each rod is 1.8cm, and 2.5 cm of strap is required to fasten the strap how long is the strap?

Assuming the seven rods are arranged such that the 7th rod is surrounded by the other 6, then the strap must have a length equalling 2.5 cm + 6 * 1.8 cm + 2 * pi * (1.8/2) ^2 cm which is

13.3 + pi * 1.62 cm or approximately 18.4 cm to 1 decimal place.