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### Course: Digital SAT Math>Unit 3

Lesson 9: Data inferences: foundations

# Data inferences — Basic example

Watch Sal work through a basic Data inferences problem.

## Want to join the conversation?

• An easier way: Cross multiply, do 399 x 635k then divide all over 1500. the number in your calculator becomes 168, 910 which is closest to D
• I agree and tried the same cross multiply method and I got the same answer, if this on the calculator ok part it would have taken a few seconds to solve.
• Honestly, with a calculator, I found it easier to just multiply the proportion(399/1500) times the population (635,000)
• Will Sal take away the old SAT practice after January?
• I doubt it. The old SAT has its values and I wouldn't think Sal would remove all the work he did with making videos of all the practice problems.
• how do you know to set it up like that? 399 over 1500?
• If we want to get more technical and forget about Sal's way of solving the problem, it's not really important whether it's 399 over 1500 or 1500 over 399 (although 399 over 1500 makes more sense) the important part is that when you know which belongs with what.

1500 is with 635000, because both of them represent the whole group, so:

133/1500 = x / 635000
And to solve for x:
133/1500 * 635000 = x (this is where 133/1500 * 635000 came from)

Likewise, if you somehow decided to use 1500/133, then:
1500 / 133 = 635000 / x (remember, 1500 and 635000 must be in the same level, in this case numerator)

Solve for x:

1500/133 * x = 635000 (first we bring x to the top by multiplying both sides by x)

x = 635000 * 133/1500 (then solve for x, like a normal equation)

And if you have noticed, both methods lead to 63500 * 133/1500, but the first one is easier (in this case) and makes more sense. In ratios and percents, we usually write part / the whole.

Hope this helped.
• Can we use a calculator for this part? Because that would make finding the answer super quick here.
• why is the example always easier than the task?
• YO ts goofy ion rlly fw dis luh bum ah video i fw da other 1 heavy tho
• We cannot understand your sentence
• At , Sal sir rounded off 399, but in case if the options are not too much ranging so is it ok to this ronuding off thing.
• Yes. Notice that it says approximately, so you will have to round anyway.
400/1500 = 0.2667
399/1500 = 0.266
That is a difference in the ten thousandth place, so you buy a LOT of time by rounding and you do not lose much at all in accuracy.
The "real" answer using 399/1500 is 168,910
and the estimated answer is 169,333.333
The `estimated` answer they gave in the options is 169,000 with the closest contender 127,000 which is way too small to be tempting.
So, your rounding doesn't cause you to lose anything, except extra seconds or minutes of messing around with the "real" numbers that are harder to work with.