This is an online community of students and teachers. Students can post their questions which might be exam questions or generic doubts. Teachers help the students with solutions and proper guidance.
What to do
Be courteous - Being a diverse community of people, we must show respect to each other-s opinions. This will enhance the level and quality of interaction.
Language - While posting content, we must use simple and easy to understand language. Avoid using chat lingo.
Content - Each one of us possess a vast pool of knowledge gained from our academic qualifications and experience. The same originality should reflect in our answers. Do not to copy/paste the content from internet. To back up your information, cite sources wherever necessary. Take care of punctuation like question mark, comma etc. Don-t post abusive content as it may lead to suspension of your account.
Participation - Make sure that your question is placed in the right category and right discussion. This will help us suggest you relevant posts to you. As a community member, you equally share the responsibility to keep the discussion clean. Good answers should be boosted by giving thumbs-up and bad answers must be removed by reporting abusive.
Profile - As a participant, you should have an up-to-date profile so that other participants know whom they are speaking to. It also brings a trust factor in students asking questions.
What not to do
Marketing - You should not promote yourself in your answers by giving contact details. Any such posts are liable to be penalized.
Duplicate Content - After posting a question, wait for the answer. Dont post your query multiple times. Duplicate questions as well as any other form of content will be reported abusive and removed from the site.
Copyright material - Posting copyright material is strongly discouraged. Any legal implications will be borne by the person posting the same.
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According to me the answer is 5 because repetition is not allowed so, 5!/4!=5