If you're muggle-born how do you get your acceptance letter to Hogwarts if it has 'muggle repelling charms' on it?
How, for example did Hermione Granger get her letter and how did she reply to it?
JKR answered this in an interview in 2004:
How are Muggle parents convinced to let their kids go to Hogwarts, a strange place they never heard of before; and wouldn’t they think it was a practical joke?
In the case of Muggle parents, special messengers are sent to explain everything to them. But don’t forget that they will have noticed that there’s something strange about their child for the previous ten years, so it won’t come as a complete bolt from the blue.
We also see this in one of Snape’s memories, where he explains the Hogwarts letter to Lily:
Snape: ...we'll get the letter, you and me.
Lily: And will it really come by owl?
Snape: Normally. But you're Muggle-born, so someone from the school will have to come and explain to your parents.
Two other instances about admissions letters are somewhat interesting here:
Harry is a half-blood, but is being raised by two Muggles who reject the idea of magic (even if they begrudgingly accept its existence). His first letter is sent through the Muggle post, and when it’s clear that his letters aren’t being received, Hagrid gets dispatched to straighten things out.
Lupin is also a half-blood, and grew up with both of his parents, but he had his werewolf bite before he got his Hogwarts letter. Both of them assumed he wouldn’t be able to attend, but Dumbledore himself paid them a visit to assure them that he could attend.