Consider the question. Daria wants to go to a concert by her favorite band. She wants to buy tickets for her and for three of her friends. One ticket cost is $90. How much money does Daria need to earn if she currently has only $189?
 If the answer is '171'; explain the reasoning:
Daria wants to buy tickets for four persons, so the cost will be 4 tickets * $90 / ticket = $360. Daria currently has $189, so she needs $360 - $189 = $171 more.