Answer:
(d) 32
Step-by-step explanation:
An estimate is a value that is intended to approximate the result of a detailed calculation. Often, the target is an estimate that is within about 10% of the actual value. A "rough order of magnitude" (ROM) estimate is intended to be within about a factor of 3 of the actual value.
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An estimate is generally made by using a reduced number of significant figures in the calculations. Often, the numbers used will be rounded to 1 or 2 significant figures. The math is often accomplished mentally, so methods of mental arithmetic are often used to simplify the calculations.
rounded values
We are asked for an estimate of the product (-24.98)(-1.29). We notice there are an even number of minus signs, so the result will be positive. The values are (25)(1.3) when rounded to 2 significant figures.
Along with the rounded values, it is often useful to make note of the direction and magnitude of the errors due to rounding. Here, we have rounded both numbers up. The number 25 is high by 0.02, and the number 1.3 is also high by 0.01. These can be used to estimate the error in the estimated value.
the product
The desired product can be computed several different ways. Either or both of the numbers 25 and 1.3 can be represented in a fashion that facilitates the (mental) arithmetic. Here are three (3) methods that could be used.
- (25)(1.3) = 25(1 +0.3) = 25 +7.5 = 32.5 (a little high, so we can choose to round down to 32)
- (25)(1.3) = (100/4)(1.3) = 130/4 = 65/2 ≈ 64/2 = 32 (again, rounding down because we know 32.5 is too high)
- (25)(1.3) = 2.5(13) = 2(13) +13/2 = 26 +6.5 = 32.5 ≈ 32
Ryder's estimate was 32.
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Additional comment
The actual product is ...
(25 -0.02)(1.3 -0.01) = (25)(1.3) -0.02(1.3) -0.01(25) +(0.02)(0.01).
As a first approximation of the error in the product 32.5, we can sum the middle two terms:
-0.02(1.3) and -0.01(25) = -0.026-0.25 = -0.276.
Then the first refinement of the estimate will give us ...
32.500 -0.276 = 32.224
The actual value is obtained by adding the final error term:
32.224 +0.0002 = 32.2242 . . . . the actual product with estimate errors resolved.
