Next consider multiplication. Following the previous example, suppose that we want to know the area of a particular rectangular region on the table-top. With the meter stick we again measaure the length of one side of the rectangle to be 0.95 meters, and with the more accurate ruler we measure the other side to be 0.217 meters. The first number only has two digits, the second number has three, and the result has 5 digits. Again, notice that when we restricted the answer to the two good digits we round , not just truncate.
Similar consideration apply to division.
When multiplying or dividing, perform the operation as usual, but restrict your result to the smallest number of digits from the beginning of the number in any operand. They are not a replacement for a careful analysis of the propagation of errors through a calculation, but they are usually enough for most practical purposes. When adding or subtracting, perform the operation as usual, but restrict your result to the smallest number of digits past the decimal in any operand. When multiplying or dividing, perform the operation as usual, but restrict your result to the smallest number of digits from the beginning of the number in any operand.