January 12th, 2013


Bad Math

Quiz time: I was trying to read a building schematic yesterday, and there was an interior wall that was 7 3/8 inches long on the blueprint. In the upper-right corner of the page, it said 1/4" to 1'.

Question: how long is the wall in real life?

Answer: scroll down to see. Or, give the answer in the comments, along with an explanation of how you got it

The teacher gave out a one-page sheet "explaining" how to do the problem. Here is the entire method he wanted us to use (copied verbatim)


Calculation 1
This calculation deals with the whole number measurement. In this example, the whole number is 7.

The formula is
4 feet x ____ = ____

Therefore, the calculation is
4 feet x 7 = 28

Calculation 2
Before presenting the calculation, here are some charts that the calculation will use:

Partial Measurement Translation
If the measurement equals Then the Actual Footage equals
1/2 inch = 24 inches or 2 feet
1/4 inch = 12 inches or 1 foot
1/8 inches = 6 inches or 1/2 foot
1/16 inches = 3 inches or 1/4 foot

Handy Guide
If the calculation result is Then the Actual Footage equals
12 inches = 1 foot
24 inches = 2 feet
36 inches = 3 feet

The formula is
1. Check the measurement for the partial number. In this example, the number is 3/8.
2. Check the Partial Measurement Translation for the amount that the bottom part of the fraction matches (denominator). In this example, the number is 8 and the chart says that 8 equals 6 inches.
3. Check the partial number for the top part of the fraction (numerator), In this example, the number is 3.
4. Multiply 6 (the chart translation for the bottom part of the fraction) by the top part of the fraction which is 3. The answer to 6 x 3 is 18 inches.
5. Check the Handy Guide. 18 inches is bigger than 12 inches but smaller than 24 inches. Therefore, the measurement equals 1 foot 6 inches or 1 1/2 feet (18-12 = 6 inches and the 12 inches = 1 foot)
6. Here is the calculation in numerical format:
a. Bolded Step 2 x Bolded Step 3 = Step 4. (6 x 3 = 18)
b. Handy Guide Smaller Number - Step 4 = A complete foot/feet measurement and a remainder.

Calculation 3
This calculation provides the final answer.

The formula is
Result of Calculation 1 + Whole Number Result of Calculation 2 + Remainder Result of Calculation 2

28+1+6 or 29 feet and 6 inches or it may be expressed as 29 and 1/2 feet.


Rant: Are you kidding me?!?!?! Are you FRICKING KIDDING me? This is the way they teach kids to do math? I hope you realized that the whole problem is just multiplying 7 3/8 by 4.

Now, I understand that a lot of high school graduates may not be good at math and don't understand fractions. But judging from the above, I can understand how someone could develop a huge FEAR of mathematics. It definitely doesn't promote understanding the calculation. The instructor's answer took seven steps and TWO LOOK-UP TABLES! There is no reason that this problem should require two look-up tables.

It turns out talking to some construction workers that some guys keep a cheat sheet with their tools listing the most common wall lengths and how long they would be on a blueprint. Or, if something is 7 3/8", they just cut the wood at 28' and add "a little extra". But I can imagine with a foreman yelling at you and knowing any mistake could cost the company lots of money, the measurements could get really screwed up.

Maybe we should all try to go metric and then give everyone calculators.
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