Ian won $100 in the lottery.  He decided to use the money to pay off debts. He paid $20 to Colin. He then paid twice as much to Helen, as he had paid to Colin. Then finally, he paid half as much to Benedict, as he had paid to Helen.  How much money, in dollars, does he have left after paying off debts?
Starting with $100, he paid $20 to Colin, leaving him with $100-$20=$
<gadget id="calculator">100-20</gadget>
<output>80</output>
80.
Twice as much as $20 is 2*$20=$
<gadget id="calculator">2*20</gadget>
<output>40</output>
40.
Thus, he paid $40 to Helen, leaving him with $80-$40=$
<gadget id="calculator">80-40</gadget>
<output>40</output>
40.
Half as much to Benedict as he had paid to Helen is $40/2=$
<gadget id="calculator">40/2</gadget>
<output>20</output>
20.
Thus, he paid $20 to Benedict, leaving him with $40-$20=$20 after paying off all debts.

<result>20</result>