Test 1 Section 7 - #17 (page 418)

They tell us these are overlapping equilateral triangles, and that CD, DE, and EF are all 10 inches long, so let's just fill in what we know first (the fact that they're equilateral and thus have angles that all equal 60° is very important here).

(This is totally not to scale.  Sorry.  Drawing with a mouse is hard!)  

Note that the little triangle in the middle is also equilateral.  We can use that to fill in all our distances.  The bases of the big triangles (CE and DF on your diagram) are both 20, so all the sides of the big triangles will be 20.  The smaller triangle will have sides of 10 inches each.  That means we're cutting the overlapping sides in half:

To find our perimeter and finish the question, just count up all the sides that are bolded.  They add up to 90.