Focus on the smallest triangle and the medium sized triangle. We can form the proportion 2/4 = x/5 by dividing the corresponding sides together.
Solving for x leads us to the following
2/4 = x/5
2*5 = 4*x
10 = 4x
4x = 10
x = 10/4
x =5/2
x = 2.5
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Now focus on the medium sized triangle and the largest triangle. We'll use the same idea as before
4/8 = 5/y
4y = 8*5
4y = 40
y = 40/4
y = 10
Note how each horizontal line is a midsegment, which means that jumping from y = 10 to 5 is us taking half of that value. Then going from 5 to x = 5/2 = 2.5 is another step of cutting in half.
