There is no particular way by which you could avoid your calculation mistakes in your examination. Mistakes are made, you are a human being. But there can something you can do.Be cautious and be alert.While calculation,make sure that recheck the question first and calculate. Especially in lessons like Mensuration.
Sometimes, you can easily understand your mistake when you do not get the expected answer or sometimes the answer comes in decimals which are really huge.That shows that something is wrong and you need to check your answer.
Keep in mind, the time management key.Its the key to success.You should get time to recheck your paper once.You might have done some very silly mistakes! Like once I had written 6 × 6 = 6 (That's really silly ! I was in hurry)
So, go ahead write your mathematics paper and Best of Luck!