BLAHBLAHUN: Dawn will get $780 more if her salary is increased by 12%. Dawn's colleague Su Lin's salary is 9% more than Dawn. What is Su Lin's salary?
We know two things,
1) 780 is 12% of Dawn's salary
2) Su lin earns 9% more than Dawn
We can calculate Dawn's salary by computing the following.
We know $780 is 12% of the salary so 1% of Dawn's salary is 780/12 = $65
We know $65 is 1% of the salary so 100% of Dawn's salary is $65*100 = $6500
Now Su lin earns 9% more than Danw so
Su lin's salary is $6500 + 9% of $6500 = $6500 + 9/100 * $6500 = $6500 + 0.09*$6500 = $7085
Now I computed this in a very slow and precise matter for the purpose of explanation but we can also do the following.
780/0.12 * 1.09 = $7085
Why?
Look at the following;
if we wanted to know 12% of $6500 we would have multiplied $6500 by 12/100 (which is 0.12)
Now similarly if we want to know the opposite (so we know 12% is 780 and we want to know 100%) we can divide by 0.12 instead of multiply (since division is the opposite of multiplication)
Now for the multiplication by 1.09, we know that Su lin has 9% more salary.
So as I showed before Su lin's salary is $6500 + 9% of $6500 = $6500 + 9/100 * $6500 = $6500 + 0.09*$6500
which gives 6500 + 0.09*6500 = (1+0.09) * 6500 = 1.09 * 6500 = 1.09 * 780/0.12
If you want to know more about percentages, these might help;
http://www.mathsisfun.com/percentage.html
http://www.mathsisfun.com/decimal-fraction-percentage.html
Reinout
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