Math homework question 5th gradeon estimating when multiplying decimals?

Question

Math 5th grade how to estimate when multiplying decimals?
My daughter (and I) are confused about rules for estimating when multiplying decimals. The example is 2.4 x 45.2. I thought estimating would be 2 x 45 (given the .4 and .2 is below .5 which then you would round down). The teacher listed rules, in first underlining the largest place value(for 2.4 would be 2; 45.2 would be 4); second round the numbers in 2.4 becomes 2 and 45.2 becomes 50. third mulitply 2 x 50. He then gives 3 examples to round to the largest place: 18.002, 346.2 and 23,453. What would the answers be and can you give any other hints on how to learn this?

hayharbr · Accepted Answer

The largest place means the place of the first digit that is not a zero.  So for 18.002 it would be the nearest ten so 20;,346.2  would be nearest hundred so 300, and 23,453 is nearest ten thousand so 20,000.

The teacher wants you to find the roughest, and therefore easiest to do in your head, estimate.  That's why he rounded 45.2 off to 50 and not 45; it's easier to do 50 x 2 in your head (though not much!)

Roni · Answer

I would tell you, but hayharbr already said what I was going to say. It would be 20, 300, and 20,000

Kanye · Answer

go 20 350 and 25,000 that should be right

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Math homework question 5th gradeon estimating when multiplying decimals?

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