Julia went to the movies and bought one jumbo popcorn and two chocolate chip cookies for $5.00. marvin went to the same movie and bought one jumbo popcorn and four chocolate chip cookies for $6.00. how much does one chocolate chip cookie cost?

X=price of one jumbo popcorn
y=price of one chocolate chip cookies

$5.00=$5.00(100 cts / $)=500 cts
$6.00=$6.00(100 cts / $)=600 cts

We suggest this system of equations:

x+2y=500
x+4y=600

we solve this system of equations by reduction method.

-(x+2y=500)
  x+4y=600

      2y=100    ⇒ y=100/2=50

x+2y=500
x+2(50)=500
x+100=500
x=500-100
x=400

solution: one chocolate chip cookie cost 50 cts.