All About Percentages : GMAT Quantitative Section
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12 Oct 2013, 04:34
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The detail concepts of the Percentage is collected from various source. I guess it will useful for others.

What is a Percentage?

A fraction with denominator 100 is called percentage. To convert a fraction into percentage, multiply by 100 and put % sign.

Expressing one quantity as percentage of other:

Ex: What percent is number 5 of number 25?
Out of 25-> 5,
Out of 1-> 5/25,
Out of 100-> (5/25)*100= 20%.

Percentage Change:

General Formula: Percent change=$$((Change in Quantity)/(Original Quantity)*100)$$

Percentage increase=$$(Increased value-original value)/(Original value)*100$$

Percentage decreased=$$(Original value-Decreased value)/(Original value)*100$$

If a value R is increased by x%, then decrease the increased resultant value $$(R+Rx/100)$$ by $$(x/(x+100)*100)%$$ to get back the original value R.

If a value R is decreased by x%, then increase the decreased resultant value$$(R-Rx/100)$$ by$$(x/(100-x)*100)%$$ to get back the original value R.

Ex: 1000 is increased by 20% then resultant value is 1200.

20/120*100=16.67% (Rounded to nearest digit)

So, 1200 is reduced by 16.66% = 1200*16.67/100=200.04

1200- 200.04= 1000 – the approximation due to the round off- If we use 16.66666666666667 we will get the exactly 200.

Percentage Increased/ Reduced BY and Percentage Increased/ Reduced TO:

If a quantity is reduced by x% then result will (100-x)% of orginal, and if a quantity is reduced to x%, then the new value is x% of original value.
By represent difference and to represents the final value.

Ex: if a rate of product is reduced by 30%, then the new rate will be 70% of original, and if the rate of the product is reduced to 30%, then new rate will be 30% of original.

Percentage in POPULATIONS:

If the original population of region is A, and annual growth is x%. Then population after R years is $$A(1+x/100)^R$$.

Increase in Population –$$A[(1+x/100)^R-1]$$

If the original population of region is A, and decrease in population is x%. Then population after R years is $$A(1-x/100)^R$$.

Decrease in Population- $$A[1-(1+x/100)^R]$$.

If a value of a number is first increased by R% and then decreased by R%, the net change is always a decrease or loss in original value.

Hence, % Loss or decrease = $$(R/10)^2%.$$

Other Important Notes:

If a value R is increased by x% to S, and S is again decreased to R by y%, Then x is always greater than y (Positive values only).

If a value R is decreased by x% to S, and S is again increased to R by y%, then x is always less than y.

If a value P is increased by x% then by y% and then by z%, the final value will be same even if we reverse the order of x,y,z. I.e., P is increased by z% first, then by y% and then by x%. (Same for the decrease)- Successive increase or decrease in value.

Similarly if value P is increased by X% then by Y% and then decreased by Z%. the final value will be same when P is decreased by Z% first, then increased by Y%, and then by X%.

If there is an increase of x/y in any value A, then the increased value will be $$A (1+x/y)$$.

If there is a decrease of x/y in any value A, then the increased value will be $$A (1-x/y)$$.
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12 Feb 2017, 18:11
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