I have 450 grams of salt and flour mix. How many grams of flour should I add to reduce the percentage of salt in the mixture to 90% of what it was?
(a) 50
(b) 10
(c) 30
(d) 45
(e) 60
You want to add x grams of flour to the 450 grams of salt. Thus you will have a total mass of 450 + x grams. This will make the amount of salt be 90% of the total so:
$\displaystyle \frac{450}{450 + x} = \frac{90}{100}$ <-- Cross multiply
$\displaystyle (450)(100) = (90)(450 + x)$
$\displaystyle 45000 = 40500 + 90x$
$\displaystyle 45000 - 40500 = 90x$
$\displaystyle 4500 = 90x$
$\displaystyle \frac{4500}{90} = x$
$\displaystyle x = 50$
So we need to add 50 grams of flour.
-Dan