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Re: The earth travels around the sun at a speed of approximately 18.5 mile
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30 Aug 2015, 03:10

18.5miles/second x 60seconds/minute x 60minutes/hour = 18.5(60)(60) = 18.5(3600) = 185(360). Long multiplication to solve.

Is there a quicker way?

My initial method, as I try to avoid long multiplication was: 185 x 100 = 18500 20,000 x 3 = 60,000 - 4,500 = 55,500 185 x 60 = ~9300 (185x50)+ 1850 (185x10) = 11,500 45,000 + 11,000 = 66,000

However this is somewhat in the middle of C and D and therefore prone to error. I was even a little more liberal in my rounding while doing it the first time which placed my answer right in the middle of the two. Doing this also takes a toll on my brainpower and I think I should avoid it as it tires me out a bit, which is bad in the exam. It also ended up taking as long as a quickly done long multiplication.

Would you guys suggest just going for long multiplication? Or do you have a nicer, fast and accurate way to go through this?

This is a classic case of approximations as the options are too widespread.

18.5 mps to 18.5 mph with 1 h = 3600 seconds.

Thus 18.5 mps = 18.5*3600 = a bit more than 18*3600 = a bit more than 18*18*200 = a bit more than 324*2*100 = a bit more than 64800 . So eliminate C and mark D is the correct answer.

You need to remember squares from 1 to 20 for GMAT.

This is a classic case of approximations as the options are too widespread.

18.5 mps to 18.5 mph with 1 h = 3600 seconds.

Thus 18.5 mps = 18.5*3600 = a bit more than 18*3600 = a bit more than 18*18*200 = a bit more than 324*2*100 = a bit more than 64800 . So eliminate C and mark D is the correct answer.

You need to remember squares from 1 to 20 for GMAT.

Hope this helps.

This is great! I'm going to practice this method. However I could still see a problem if the answers were, say, 65,000 and 66,600. I guess in this worst case scenario you could go back and calculate that 0.5 x 3600 and add it, which doesn't take that long.

Re: The earth travels around the sun at a speed of approximately 18.5 mile
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11 Sep 2018, 11:20

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Another way to deal with calculating (18.5)(3600) is to use the answer choices to our advantage.

First off, there's a nice rule that says:AB = (2A)(0.5B) This strategy (which is covered in the video below) can sometimes save us some calculations.

When we apply therule, (18.5)(3600) = (37)(1800) [I doubled 18.5 to get 37, and I halved 3600 to get 1800 ] = (37)(18)(100) [I rewrote 1800 as 18 x 100] = (----6)(100) [We know that the units digit of 37 x 18 must be 6, since 7 x 8 = 56] = ---600[some number that ends with 600]

Check the answer choices.... A. 1,080 B. 1,160 C. 64,800 D. 66,600 BINGO!! E. 3,996,000