Nobody's head is perfectly round. It is a free-form shape. Everybody's is a different shape.
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you cant you have to use your head but the answer is 5.35
0.89
The idea is to round each of the numbers - in this case to the nearest multiple of 10 - then do the addition in your head.
You take the number before the number you want to round it to. Round to tens: 97112770 (7 is greater then 4, so you round up) round to hundreds: 97112800 (6 is greater then 4 so you round up) round to thousands:97113000 (greater then 4, round up) round to ten thousands: 97113000 (greater then 4, round up) round to hundred thousands: 97100000 (less then 4, round down) round to millions: 97000000 (less then 4 round down) round to billions: 10,000,000 (more then 4 round up)
You round off numbers for several reasons. First, your final answer is no more accurate than your original measurement. If you do not round off, you will give an inaccurate picture of your measurement. Second, you round off numbers to check electronic equipment. You can not begin to add, subtract, multiply, and divide in your head as fast as an electronic piece of equipment. Still, by rounding off numbers, you can get a rough idea of what the answer should be. If the answer the computer gives is nowhere near your answer then something could be drastically wrong with some spread sheet formula. You round off money real fast to know if you have been cheated.