Math Word Problems Demystified - A Self Teaching Guide

Since the total amount of the mixture is 20 pounds, the first equation is

xþy¼ 20

Since the cost of the mixture is $3.75, the second equation is

4 xþ 3 y¼ 20 ð 3 : 75 Þ

IMPLEMENTATION: Solve the system

xþy¼ 20

4 xþ 3 y¼ 20 ð 3 : 75 Þ

Solve the first equation forx. Substitute in the second equation and

solve fory.

xþy¼ 20

xþy
y¼ 20 
y

x¼ 20 
y

Substitute:

4 xþ 3 y¼ 20 ð 3 : 75 Þ

4 ð 20 
yÞþ 3 y¼ 20 ð 3 : 75 Þ

80 
 4 yþ 3 y¼ 75

80 
y¼ 75

80 
 80 
y¼ 75 
 80

y¼
 5

y


 1

¼

5

1

y¼5 pounds

LESSON 18 Using Two Equations 217