The diameter of the semicircle is (22-12)cm x 2 + 16cm = 36cm. This is because when you look at the first two semicircles from the right, you’ll find there is a “line” where the two semicircles are “in contact” with each other. So, logically, if you take the diameter of the semi-circle and subtract the length of the length “in contact”, it will equal 22cm. It should work the same for the second semicircle from the right, but the length is only 12cm, a 10cm difference from 22cm. This means that the other part the second semicircle from the right has “in contact” with the third semicircle from the right is 10cm too. As this figure is symmetrical, you add the two 10cm lengths to the 16cm and voila, you get 36cm.
Hope this helped.
— Just another P6 student