Question 1201541
<pre>
The average age of two sisters is three times the age of the younger one . in seven years' time the sum of their ages will be greater than the difference in their ages by 20 years. how old are they now? (The answer is 3 and 15)
 so , 
Let x=older sister   and y=younger sister 
x+y=3y
7+(x+y) greater than (x-y)+20 

But how can you form Simultaneous equations from this ? How is it worked out ?

You don't need simultaneous/Systems of equations for every problem, unless asked to form and/or use them.
This is solved by simply using only 1 (ONE) variable. That's ALL that's needed!

Let the younger's age be Y
Since the average of the ages is thrice the younger's age, then the 2 ages' average is, 3Y, thus
making the sum of their ages, 6Y, with the older's age being 5Y. 

I hope you're following up to this point.

Now, in 7 years'time, the sum of their ages will be 6Y + 2(7) = 6Y + 14
                                                 We then get: 6Y + 14 = (5Y - Y) + 20
                                                              6Y + 14 = 4Y + 20
                                                              6Y - 4Y = 20 - 14
                                                                   2Y = 6
                                                  Younger's age or {{{highlight_green(matrix(1,5, Y, "=", 6/2, "=", highlight(3)))}}}
As the older is 5 times the younger's age (5Y), <font color = red><font size = 4><b>the older is:</font></font></b> 5(3) = <font color = red><font size = 4><b>15 years-old</font></font></b></pre>