Question 1112650
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Forget about Ann in this problem.  She was introduced specially to confuse you.


Concentrate/focus on David and Ben only.


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For them, you have these two equations

D + B = 40
D - B =  4
--------------------Add the equations

2D = 40 + 4 = 44  ====>  D = {{{44/2}}} = 22.


<U>Answer</U>.  David is 22.  Ben is 18.
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Solved.