The answer is 40 because they're are 4 groups of ten so it would go like 10,20,30,and 40.=)
15/10 = 18-x/x is the ratio of the larger to the smaller triangle
Cross multiply:
15x = 10(18 - x)
15x = 180 - 10x
25x = 180
x = 7.2
Plug back in to check
15/10 = 1.5
18-7.2/7.2 = 10.8/7.2 = 1.5 :)
Answer:
(x + 12)(x - 3)
Step-by-step explanation:

Yes, every equilateral triangle is acute because the angles are always congruent(identical) and 60 degrees which is acute. But I may be wrong. :P