Date: May 28, 2013 11:30 AM
Author: Michael
Subject: Name this formula

Today our teacher showed us a way to solve problems like this:            
/\
./ \
../ \
8 .../ \
..../ \
...../_____\
....../ 10 \
X ......../ \
........../ \
............/__________________\
15

Find X. He posted a way that almost took him up to 20 minutes to explain but i was thinking about this

15-10=5
10/5=2
That means 8 is 2 times bigger then X
so x/2=4

But he said that I couldent use it unless i could prove it .
So is there allready a formula like this one or any way to prove it works?