Question 1156730
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Replace Lisa (MENTALLY) by other person Mona, who is 3 years younger than Liza.


Then John and Mona are of the same age, and the sum of their ages is 21-3 = 18.


Therefore, John is 18/2= 9 years old and Mona is 9 years old, too.


From this point, you immediately conclude that Lisa is 9+3 = 12 years old.


<U>ANSWER</U>.  The ordered pair (x,y) is (9,12).
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Solved.


At this point, you may forget about this person Mona, since it was "imaginary" person, introduced to facilitate the solution, ONLY.