[General] Is it a good or bad sentence?

[whsans]

Thank you for reading this thread.

Every time I drop by in this site to ask sth,
I feel sorry for myself in that I can't take part in answering to questions...;-(

I've been volunteering to teach English for some helpless students,
and I'm trying to teach them my best.
To be honest, I'm not that good in English,
so I have difficulty many times in teaching them.

My question is the following sentence.

A washing machine does in an instant what used to take people hours to do.

It looked natural at first sight, but while reading it over and again,
I realized sth is weird, especially the following part.

What used to take people hours to do

I had a feeling that the above sentence needs 'it' btwn 'what' and 'used'
so I fixed the sentence as follows.

A washing machine does in an instant what it used to take people hours to do
(it means not 'the washing machine', but 'to do'

but I can't assure that the original sentence is wrong because I don't know why. I asked another volunteer who is a native English speaker, and she said the original one looks bad, but she doesn't know why.

let me straighten my questions.
Which one is correct, the original sentence or the fixed one?
Or both of them are correct?

Thanks again for reading my question.

 

[emsr2d2]

Thank you for reading this thread.

Every time I drop by in this site to ask sth,
I feel sorry for myself in that I can't take part in answering to questions...;-(

I've been volunteering to teach English for some helpless students,
and I'm trying to teach them my best.
To be honest, I'm not that good in English,
so I have difficulty many times in teaching them.

My question is the following sentence.

A washing machine does in an instant what used to take people hours to do.

It looked natural at first sight, but while reading it over and again,
I realized sth is weird, especially the following part.

What used to take people hours to do

I had a feeling that the above sentence needs 'it' btwn 'what' and 'used'
so I fixed the sentence as follows.

A washing machine does in an instant what it used to take people hours to do
(it means not 'the washing machine', but 'to do'

but I can't assure that the original sentence is wrong because I don't know why. I asked another volunteer who is a native English speaker, and she said the original one looks bad, but she doesn't know why.

let me straighten my questions.
Which one is correct, the original sentence or the fixed one?
Or both of them are correct?

Thanks again for reading my question.

The "fixed" sentence sounds better "...what it used to take..."

If you imagine the sentences reversed, you get "It used to take people hours to do the washing, but now the washing machine does it in an instant (or at least in a very short time!)"

 

[whsans]

I really appreciate your kind answer.
It really gave me a big help.
Have a great, great day