Answer:
Let's call the weight of the large boxes:
l
Let's call the weight of the small boxes:
s
From the information in the equation we can now write two equations:
Equation 1:
5
l
+
3
s
=
99
Equation 2:
7
l
+
9
s
=
171
Step 1) Solve each equation for
9
s
:
Equation 1:
5
l
+
3
s
=
99
5
l
−
5
l
+
3
s
=
99
−
5
l
0
+
3
s
=
99
−
5
l
3
s
=
99
−
5
l
3
×
3
s
=
3
(
99
−
5
l
)
9
s
=
(
3
×
99
)
−
(
3
×
5
l
)
9
s
=
297
−
15
l
Equation 1:
7
l
+
9
s
=
171
7
l
−
7
l
+
9
s
=
171
−
7
l
0
+
9
s
=
171
−
7
l
9
s
=
171
−
7
l
Step 2) Because the left side of both equations are equivalent or the same we can equate the right side of each equation and solve for
l
:
297
−
15
l
=
171
−
7
l
297
−
171
−
15
l
+
15
l
=
171
−
171
−
7
l
+
15
l
126
−
0
=
0
+
(
−
7
+
15
)
l
126
=
8
l
126
8
=
8
l
8
15.75
=
8
l
8
15.75
=
l
l
=
15.75
Step 3) Substitute
15.75
for
l
in the solution to either equation in Step 1 and solve for
s
:
9
s
=
297
−
15
l
becomes:
9
s
=
297
−
(
15
×
15.75
)
9
s
=
297
−
236.25
9
s
=
60.75
9
s
9
=
60.75
9
9
s
9
=
6.75
s
=
6.75
The Solution Is:
The large boxes weigh 15.75 Kg
The small boxes weigh 6.75 Kg