Question 161004
Let x=amount of $1.40 jelly beans needed
Value of the $1.40 beans=$1.40x
Value of the $2.40 beans=$2.40(7)
Value of the final mixture=$2.10(7+x)
Now we know that the value of the beans before they are mixed together has to equal the value of the beans after they are mixed together, so:
$1.40x+$2.40*7=$2.10(7+x)  get rid of parens (distributive law)
$1.40x+$16.80=$14.70+$2.10x subtract $14.70 and also $1.40x from each side
$1.40x-$1.40x+$16.80-$14.70=$14.70-$14.70+$2.10x-$1.40x collect like terms
$2.10=$0.70x  divide each side by $0.60
x=3 lbs-------------amount of $1.40 jelly beans needed

CK
$1.40*3+$2.40*7=$2.10*10
$4.20+$16.80=$21.00
$21.00=$21.00

Hope this helps--ptaylor