Harriet has 3/4 tank of gas left in her car. if she needs 1/4 tank to go to her friends house and another 1/4 tank to get back home， does she have enough gas? explain why

Yes she does have enough. Here is why: If you add 1/4 and 1/4, you get 2/8 which is also 1/2. 3/4 is bigger than one half. If you subtract 1/2 from 3/4, you get 1/4. So she would have 1/4 left. Absolutely enough to get to her friends house and back. :)