lucy1988 said:
well i think as 70% of 45 is about 31 percent ...
The percentage of a number is not another percent; it's another number.
In other words, we say that:
70% of 45 is "about 31", not "about 31 percent".
(Maybe you typed it that way due to a subconcious slip-up; if so, I understand how
that goes. :wink: )
lucy1988 said:
... i must have got about 45% in the exam (?) ...
Yes -- that's a good rough estimate. I'm assuming that you still would like to calculate the exact exam score ...
lucy1988 said:
... but just cant get my head around like, accuracy?
Then use a calculator.
Really, I'm serious. The symbol '%' can be thought of as a mathematical operator that means 'divide by 100'. So, if you're using a calculator without a '%' button, then 70% is entered as 70/100.
Perhaps you remember that dividing a number by 100 is the same as moving the decimal point two places to the LEFT.
70 with a decimal point is 70.0
Moving that decimal point two places to the left yields 0.70
So, 70% is 0.70 on the calculator (as well as everywhere else we might need a decimal form of 70%).
You may hear people talk about the "decimal form" of a percent. That's just the decimal number that you get after dropping the percent sign and moving the decimal point two places to the left (i.e., the percent written as a decimal).
The decimal form of 34.5% is 0.345
The decimal form of 250% is 2.50
We always use the decimal form of percents when we do arithmetic with those percents.
In Seattle, we pay 9.5% sales tax at restaurants.
So, the sales tax on my $5 footlong at Subway is 5.0 times 0.095, or 48 cents.
70% of 45 is calculated using the decimal form.
45 * 0.70 = 31.5
That's accuracy!
lucy1988 said:
... can anyone figure out what mark i got in the exam?
Yes -- but most of us are motivated more to guide you to do it yourself.
lucy1988 said:
i need to figure out how badly i've done in a psychology exam urgently ...
I hope it's not a life-or-death situation! :wink: I'll work through a similar problem, and perhaps you can then work through yours.
Out of 100 possible course-grade points in my Communications-101 class, I earned a total of 78%.
15% of my course grade is based on my total homework points (which I pretty much aced). I got 98% of those 15 possible points.
25% of my course grade is based on my written exam. I don't yet know how many of these 25 possible points I earned from my exam score. This is what I want to calculate first, so that I can use it to determine the actual exam score.
The remaining 60% of my course grade is based on my oral presentation in front of the class -- which I botched because I could not stop laughing. Out of those 60 possible points, I got only 20% of them.
I'll follow Elizabeth's lead ...
98% of the 15 homework points is
15 * 0.98 = 14.7
(Homework contributed 14.7 of the 78 total points that I earned in the class.)
20% of the 60 oral-presentation points is
60 * 0.20 = 12
(My giggling contributed 12 of the 78 total points that I earned in the class.)
Since my total points for the class are 78, the points earned from the written exam must be 78 less both the 14.7 and the 12.
78 - 14.7 - 12 = 51.3
(The written exam score contributed 51.3 of the 78 total points that I earned in the class.)
Now that I calculated these 51.3 points, I can calculate the exam score percentage because THAT percentage of the 60 possible points is 51.3.
In other words, I'm now asking myself, "51.3 is what percent of 60?".
I find that percent by dividing 51.3 by 60. (More on this at the end ...)
51.3 / 60 = 0.855
Guess what? To change the DECIMAL form of a percent back to an expression WITH A '%', move the decimal point two places to the RIGHT and add the percent sign.
0.855 = 85.5%
My written exam score is 85.5%
Anytime you want to answer the question: "Number1 is WHAT PERCENT of Number2?", then divide Number1 by Number2 and change the decimal result back into an expression with a percent sign.
33 is what percent of 144?
33 / 144 = 0.2292
33 is 22.92% of 144.
6 is what percent of 24?
6 / 24 = 0.25
6 is 25% of 24.
416 is what percent of 200?
416 / 200 = 2.08
416 is 208% of 200.
If you still need help to calculate your psychology exam after working through my example, then please post specific questions about what it is exactly that you don't understand. Also, please show your work.
Cheers,
~ Mark
