In daily oral communication, there is no need to add colons to the answers to math application questions. If you are writing a formal document or paper, especially when solving a math problem, you usually put a colon before the answer to remind readers that it is an answer rather than descriptive text. But if you just chat with friends or discuss topics with classmates, adding or not adding colons will not affect the clarity of communication. In formal occasions, remember to pay attention to the format specification and write clearly the words that meet the requirements of document typesetting.