The full price is represented by 100%. If this computer was decreased by 20%, so it worth 80% now.
So, we can make a rule of three: 80% value is equals to 136, how much is 100%?
80% ------------ 136
100% ----------- x
Let's multiply "making a x":
[tex]80x = 136\cdot 100\\80x = 13.600\\x = \frac{13600}{80} \\x = 170[/tex]
Therefore, the price before the decrease was £170.
