Numbers 15-16 triangles, "identify similar figures and find missing lengths"

15. 
Rectangles A and B are not similar, because 3/2.5 is 1.2, while 5/4.5 is about 1.1. This means that the two rectangles cannot be increased or decreased by any scale factors to equal each other. Rectangles A and C are not similar, because 3/10 is 0.3, while 5/18 is about 2.8. Rectangles B and C, however, are similar, because 2.5/10 is 0.25, and 4.5/18 is 0.25 as well. This means that Rectangle B can be increased using a scale factor of 4 to make it equal to Rectangle C.

16.
Yes. All squares are similar because, by definition, all the sides of a square are the same. This means that there is only one value to consider when finding out whether or not another square is similar to it. All numbers are divisible by another number, even if the quotient is a decimal, meaning that all squares are similar.