You have 100 total problems to do. you have done 51, and have 49 left. what percent of problems do you need to finish? what percent have you already done?

If you're working out a percentage, you should equate the problem out of 100. Luckily for you, this question has a total of 100 anyway, so there's not much to do.

If you've done 51/100, you've done 51%, because 51 out of 100 is already out of 100, so you don't need to change any numbers.

If you have 49 left, then you still need to complete 49%. Once again, 49 out of 100 is already of 100, so it's literally just 49%.