Find out the gramma mistake!

In the original sentence, the verb does not agree with its subject
The correct sentence should be:
Washing the cars takes Mr. and Mrs Li two hours.
“Washing the cars” is a singular subject, and it takes a singular verb, takes.
e.g. Taking care of the naughty boys is difficult.

Washing the cars (takes) Mr and Mrs Li two hours. (The subject is washing)

同意樓上說：the verb does not agree with its subject

但這句不單是這個問題，相信washing the cars是已經過去的動作，所以要用took

Washing the cars took Mr and Mrs Li two hours.

希望幫到你！

2010-05-28 14:44:18 補充：
除非washing the care這件事，每次都花李生、李太兩個鐘，若是這樣，句子會加上every time.

It may be'Mr and Mrs Li used two hours for washing the cars'
不過,在正常情況下,如果是被動式,動詞要轉為:is/am/are+pp
以上是主動式。

Washing the cars take Mr and Mrs Li two hours.

應該係:

IT TAKES 2 HOURS FOR MR. AND MRS. LI TO WASH THE CARS.

我估下先．．．

因為．．．這句有點廣東話的排法．．．
如果沒有估錯，意思應該是李先生和李太太用了兩小時洗車，
比較簡單直接應該是 Mrs and Mr Li took two hours to wash the cars.
首先應該是 Lady first 吧，嘻！
另外應該是過去式，因為洗好了才知道用了多少時間嘛！

如果硬是要用被動，應該是 The cars were cleaned in two hours by Mrs and Mr Li.
這個我感覺上覺得，用clean 好像對一點，又不是grammatical 的關係，只是自我感覺好一點，哈哈。
而被動時應該用 is/am/are/was/were + past participle ....by....，對不？

很久沒有研究這東西，謝謝你，請指教。