Ken has $15 more. If he gives this $15 to Lisa, Lisa will have $15 more than him. Now if he gives $1 more, his amount decreases by $1 while Lisa's increases by $1. So the difference increases by $2. The difference change is always twice the amount given. From $15 more to $21 more, that's a change in difference of $6. So $6 ÷ 2 = $3. He gives Lisa $3 on top of that $15, a total of $18 given.