21
look, ghost, I really don't wish to be seen as a jerk or an idiot but a 21 year old not being able to do such questions? I mean not just this one, rather all those you have posted and some of those were solved by me. I am the same age as you and I am not genius in mathematics, rather I am one of the lowest scorers in maths in my batch of engineering and I could do all of them orally.
all those questions are meant for 15-16 year old kids. Getting a decent book to solve from will help you more rather than posting here and waiting for an answer. All the questions are very basic and straightforward with no hidden tricks or something..
Do a lot of questions and keep doing them, you will learn a lot that way. And not just the number... rather the quality and variety of questions will help too.
buy a book, solve all the examples and match your solutions with the ones provided by the book then do the exercise.. that's the only way to do math.
EDIt: - if you have a teacher.. remember that all he is good for is to solve doubts.. study yourself and you will find that you have learned more
