Linear Algebra
posted by Anonymous .
Rosa ordered 120 pieces of jewelry. a number of bracelets for $10 each a number of necklaces for $12 each. She wrote check for $1400. How many bracelets and necklaces did she buy?

Let
x=number of bracelets
y=number of necklaces
then
x+y=120
10x+12y=1400
Solve for x and y.
Quick mental calculation, no paper required:
If Rosa bought only necklaces, she would have paid 12*120=$1440.
Exchanging a necklace for a bracelet saves her $2, so to pay $1400, she must have saved $40, or bought 20 bracelets and 100 necklaces.