Question 1201541
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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 ?
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j, younger
v, older


To follow the description exactly as written,
{{{system((j+v)/2=3j,(j+7)+(v+7)-(v-j)=20)}}}


Simplifying from that system,

{{{system(j+v=6j,j+v+14-v+j=20)}}}


{{{system(j+v=6j,2j=6)}}}


{{{system(v=5j,j=3)}}}