Question 526223
You're four years older than your 18 year old sister. 
My age = 18 + 4
My age = 22

Now suppose x years ago I was three times of her age

x years ago means

my age was = 22-x
my sisters age was = 18-x

then I was three times of her age

22-x=3(18-x)
22-x=54-3x
-x+3x=54-22
2x=32
x=32/2
x=16

 
16 years ago I was three times of my sisters age.