Wang Dong has a deposit of 50 yuan, and Zhang Hua has a deposit of 30 yuan. Zhang Hua wants to catch up with Wang Dong. Wang Dong saves 5 yuan a month, and Zhang Hua saves 9 yuan a month. How many months will it take to catch up with Wang Dong?
王冬有存款50元，张华有存款30元，张华想赶上王冬．王冬每月存5元，张华每月存9元，多少个月后才能赶上王冬．
<gadget id="calculator">50 - 30</gadget>
<output>20</output>

<gadget id="calculator">9 - 5</gadget>
<output>4</output>

<gadget id="calculator">20 / 4</gadget>
<output>5</output>

<result>5</result>