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  3. 15 is 60% of what number?

15 is 60% of what number?

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问题 15 is 60% of what number?
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答案25
解析 Use the decimal equivalent of 60%. Because 60% of some number z is 15, multiply z by the decimal equivalent of 60%, or 0.6.
0.6z=15
Now solve for z by dividing both sides of the equation by 0.6 as follows.
z == 25
Using a proportion, look for a number z such that

Hence, 60z =(15)(100), and therefore, z == 25. That is, 15 is 60% of25.
Although the discussion about percent so far assumes a context of a part and a whole, it is not necessary that the part be less than the whole. In general, the whole is called the base of the percent. When the numerator of a percent is greater than the base, the percent is greater than 100%. For example, 15 is 300% of 5, since

and 250% of 16 is()(16)=(2.5)(16)= 40. Note that the decimal equivalent of 250% is 2.5.
It is also not necessary for the part to be related to the whole at all, as in the question, "a teacher s salary is what percent of a banker s salary?"
When a quantity changes from an initial positive amount to another positive amount, for example, an employees salary that is raised, you can compute the amount of change as a percent of the initial amount. This is called percent change. If a quantity increases from 600 to 750, then the percent increase is found by dividing the amount of increase, 150, by the base, 600, which is the initial number given:
= 0.25 = 25%
We say the percent increase is 25%. Sometimes this computation is written as
(100%)=25%
If a quantity doubles in size, then the percent increase is 100%. For example, if a quantity changes from 150 to 300, then the percent increase is
= 100%
If a quantity decreases from 500 to 400, calculate the percent decrease as follows.
= 0.20 = 20%
The quantity decreased by 20%.
When computing a percent increase, the base is the smaller number. When computing a percent decrease, the base is the larger number. In either case, the base is the initial number, before the change.
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