Calculating percents rapidly

I guess no one understood the question. I was not asking about the 12% of 80 but rather going the other way around: "x" number when added a certain percent gives you "y" number.

Anyway, I found a clever trick, maybe some of you will find it easier, I will illustrate with an example:

The final price of a sold item is 5000 and the markup by the retailer is 25% of the cost, what is the original cost?

This of course is 5000/1.25 <- which was not my question but rather how do you calculate that number faster.

My way of thinking would be to divide 1 by its parts composed of the % (in this case 0.25).

We can easily see that .25 fits neatly 4 times in 1, so we have 5 parts in total in the denominator (4 in the "1" + 1 in the "0.25"). We can thus divide the 5000 in 5 equal parts, each being 1000.

Finally, we "lose" the part we don't want (which is 1 part or "0.25"), by simply subtracting that part from the 5000, which gives us 4000. Thus, 5000/1.25= 4000.

When dividing by a fraction (i.e. 0.5), we can apply a similar logic:

5000/0.5 <- we have 1 "parts" consisting of 0.5 and need another "part" to get to one. So, we want "2" parts in total... if one part is 5000, two parts is? 5000*2. Thus, 5000/0.5= 10,000.

Anyway, this might seem complicated to some but I find it helpful, specially for problems where an approximation is sufficient and the numbers are very ugly.

Hi eflores89,

As I mentioned in my prior post, you mind find it helpful to "convert" information to a different format. That "flexibility" to look at information in more than one way can help you to find (and take advantage of) the 'shortcuts' that the GMAT writers build into Test Day questions.

In your example, you chose to keep the information in decimal format. What if you converted it to fraction though.....?

(1.25)(X) = 5,000

Instead of having to deal with.....
X = 5,000/1.25

Try this....
1.25 = 5/4

(5/4)(X) = 5,000

X = 5,000/(5/4)
X = 5,000(4/5)
X = 20,000/5
X = 4,000

GMAT assassins aren't born, they're made,
Rich

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