Answer: 21
140% converts to the decimal form 1.40 or 1.4
You move the decimal point 2 spots to the left to convert from percent to decimal form. You can think of it as 140% = 140/100 = 1.40 since "percent" means "per 100" (think "cent" as in "100 cents in a dollar")
Now multiply 1.4 by 15 to get 1.4*15 = 21
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Side notes
1) "of" means "multiply" in percent word problems like this
2) The result 21 is larger than the original 15 because 140% is larger than 100%
3) You can optionally find the answer by solving for x in this equation shown here: x/15 = 140/100 which is of the format "part/whole = percent/100". Note how the right side 140/100 is just 1.4 in disguise, and if we multiplied both sides by 15, we end up with the same answer.