If I person $A$ & $B$ both have $40$ dollars, how much must $A$ give $B$ so that B has $10$ dollars more that $A$?
Answer is $5$ dollars. $A$ now has $35$ dollars while $B$ has $4$5 dollars. How to represent this question and answer mathematically?
This probably way off the mark but something like?:
$$40 + 40 = x + 40 + 10$$
Let A have $a$ dollars and B have $b$ dollars. When A gives $x$ dollars to B then A will have $a'=a-x$ dollars, and B will have $b'=b+x$ dollars. Now we want $$b'=a'+10\ ,$$ which translates into $$b+x=a-x+10\ .$$ Solving for $x$ we obtain $$x={1\over2}(a-b+10)\ ,$$ whatever the initial values $a$ and $b$ are. E.g., if $a=15$ and $\>b=41$ then we obtain $$x={1\over2}(15-41+10)=-8\ .$$ This means that A has to receive $8$ dollars from B in order that the "closing" condition is met.