There are 72 green pens.
There is 9 times as many white pens from 2 to 18, so there has to be 9 times as many green pens because of the ratio. 9 times 8 is 72.
Half of the distance from one side, through the middle, to the other side.
circumference: 3.14*radius*2
0.15 is bigger, cause the second digit after the decimal is bigger. You would have to add another 0.025 to 0.125 to bring it up to 0.15
