In my personal experience, there are 2 kinds of people who tend to ask such obvious questions.
- Those who are lazy and want you to do all their work.
- Those who are completely clueless and don't know how to even start working on a solution.
Not everyone always fits into those categories. Someone could have had a bad day, some stressful situation at home or have something on their mind that makes it hard to think clearly.
In either situation, you should not present them with a solution. They'll remember your helpfulness, get used to you solving their problems and ask you more frequently for help instead of working to find a solution on their own.
Instead, make it clear you won't tell them. The easiest way is to tell them
I don't remember the page, but it's written in the first half of our assignment. I'm currently working on another topic / question so you'll have to find it yourself.
Or in another situation
I don't remember the exact wording, so you should look that up in our assignment paper, in case I misinterpreted anything.
This way you are polite, indicate that you are willing to help them, but either unwilling or unable to help them with this particular question. It should also separate the lazy people from the clueless ones. A lazy person most probably notices that they cannot expect you to just hand over the solution and they either start working on their own or go bother another person. Or they might openly ask you to give them the solution, in which case you can decline more or less politely.
The clueless person is less easy to deter. In my experience, they'll often ask you to stick around and be available for the next question, and the next, and the next. They are the ones asking "What did you search for?" and "Where did you click?". People like that are often called "help vampires" because they suck you dry of energy and patience.
The best way to cope with a help vampire is to help them help themselves. Telling them
I googled for "standard letter porto"
may help them right now, but in the long run they'll be as clueless as ever. They need not learn the solution, but how to find a solution in the first place. Tell them how to break the assignment down.
We're supposed to send a letter to X. For that you need the actual letter, an envelope and a stamp. You can ask Google "How much does it cost to send a letter?" and in the answers you'll see that it's called letter porto and how much the standard letter porto is.
It will take some time, but slowly they should learn the very basics of how to break an assignment down into manageable tasks.
If they stick to their help vampire attitude, you can tell them
I already explained to you how to search for that, at least three times. I have my own work to do here and if I hold your hand at every step, I won't be able to finish my own work.
This is more than a simple "no, I won't help you". It shows them that their questions have negative consequences for other people. Some help vampires start reflecting more before asking for help, others might not.
I advice against the "asking questions back" approach. The idea is that you ask the other person questions so that the answers they give eventually answer their own question. I've seen this approach backfire spectacularly and in the worst case the asker will feel patronized and insulted.
You cannot know the train of thought of your counterpart. The questions you ask might lead them to completely different answers or simply to the reply "If I knew that I wouldn't ask you."
I've always had better results with telling people how to solve the task step by step instead of manipulating them into solving it and then expecting them to realize and remember how they did it.