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You can think of a significant digit as an "interesting" or an "important" digit that gives you useful information about a number. This means that any zeroes to the right of whole numbers or to the left of decimals can be discounted because they are placeholders. To find the number of significant digits in a number, just count the number of digits from left to right. Here are some examples:  1.239 has 4 significant digits 134.9 has 4 significant digits .0165 has 3 significant digits This depends on the problem you're working with. If you're rounding a number to two significant digits, for example, then you'll need to identify the second significant digit of the number and then use the number to the right of it to see if you should round it down or up. Here are some examples:  1.239 rounded to 3 significant digits is 1.24. This is because the digit to the right of the third digit, 3, is a 9, which is 5 or more. 134.9 rounded to 1 significant digit is 100. This is because the digit to the right of the digit in the hundreds place, or the first digit, 1, is 3, which is less than 5. 0.0165 rounded to 2 significant digits is 0.017. This is because the second significant digit is 6, and the number to the right of it, 5, makes it round up. To do this, you will first have to add up the numbers you are given. Then, you will have to find the number with the lowest amount of significant digits and then round your entire answer to that place. Here's how you do it:  13.214 + 234.6 + 7.0350 + 6.38 = 261.2290 See that the second number, 234.6, is only accurate to the tenths place, or four significant digits. Round the answer so that it is only accurate to the tenths place. 261.2290 becomes 261.2. First, multiply all of the numbers that you are given. Then, check them to see which number is rounded to the least amount of significant digits. Finally, round your finally answer to match the level of accuracy of that number. Here's how you do it:  16.235 × 0.217 × 5 = 17.614975 Notice that the 5 number only has one significant digit. This means that your final answer will only have one significant digit as well. 17.614975 rounded to one significant digit becomes 20.

Summary:
Understand what a significant digit is. Round a number to an amount of significant digits. Round to the correct number of significant digits in addition. Round to the correct number of significant digits in multiplication.