Envision Math 2.0: Grade 8, Volume 2
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3. Solve Systems by Substitution
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Exercise 15 Page 276

Practice makes perfect
We are told that all 500 seats in the auditorium are filled for a performance given by the members of a city cultural center. We are asked to find the numbers of adult tickets d and the number of student tickets s needed to be sold for the members to raise exactly $2050. d& ← Number of adult tickets sold s& ← Number of student tickets sold To determine that, we will try to form a system of equations for d and s. To fill all 500 seats, the sum of adult and student tickets must be equal to 500. d+s= 500Also, we know that the cost of an adult ticket is $6.50 and that the cost of a student ticket is $3.50. Thus, the income from the adult tickets is $6.50* d and the income from the student tickets is $3.50* s. We want the total income to be equal to $2050. Therefore, the sum of $6.50* d and $3.50* s should be equal to $2050. 6.50* d+ 3.50* s= 2050 We created two equations for d and s! Together they form a system of equations. d+s=500 & (I) 6.50d+3.50s=2050 & (II) We can solve this system using substitution. Let's begin by isolating d in the first equation.

d+s=500 6.50d+3.50s=2050
d+s-s=500-s 6.50d+3.50s=2050
d=500-s 6.50d+3.50s=2050

Now, we can substitute d=500-s into the second equation.

d=500-s 6.50d+3.50s=2050
d=500-s 6.50( 500-s)+3.50s=2050
d=500-s 6.50(500)-6.50(s)+3.50s=2050
d=500-s 3 250-6.50s+3.50s=2050
(II): Solve for s
d=500-s 3250-3250-6.50s+3.50s=2050-3250
d=500-s -3s=-1200
d=500-s s=-1200/-3
d=500-s s=400

Great! We found that the number of student tickets is s=400. Next, to find d we can substitute s=400 into either equation of the system. Let's use the first equation.

d=500-s s=400
d=500- 400 s=400
d=100 s=400

We found that d=100 adult tickets and s=400 student tickets need to be sold for the members of a city cultural center to raise exactly $2050.

This time, we are given that there were 3 times as many student tickets sold as adult tickets. Also, we know that 480 tickets were sold in total. We want to calculate the ticket sales and how much below the goal of $2050 they fell. To do that, we will first find the number of adult tickets sold d and the number of student tickets sold s. d& ← Number of adult tickets sold s& ← Number of student tickets sold Let's try to form a system of equations for d and s. We know that there were 3 times as many student ticket sold s as adult tickets d. s= 3* dAlso, we know that 480 tickets were sold in total. This means that the sum of student and adult tickets must be equal to 480. s+d= 480 We created two equations for s and d! Together they form a system of equations. s=3d & (I) s+d=480 & (II) We can solve this system using substitution. Notice that the first equations is solved for s. Thus, we can substitute s= 3d into the second equation.

s=3d s+d=480
s=3d 3d+d=480
s=3d 4d=480
s=3d d=480/4
s=3d d=120

We found that d=120. Now, to find s we can substitute d=120 into the first equation.

s=3d d=120
s=3( 120) d=120
s=360 d=120

We found the number of adult tickets sold, d= 120, and the number of student tickets sold, s=360. Now, to find the sales we need to multiply the numbers of adult and student tickets by their corresponding ticket prices. Adult tickets cost $6.50 each, and student tickets cost $3.50 each. Sales= 120* 6.50+360* 3.50 ⇕ Sales=780+1260=2 040 We found that the sales are $2040, which is $10 under the goal of $2050.