Let numbers of books be 'b' and numbers of CDs be 'c'
We can set up two equations:
Equation [1] ⇒

Equation [2] ⇒

We are solving for the number of books and the number of CDs bought
When we have two equations in terms of two different variables;

and

, that we need to solve, then this becomes a simultaneous equation problem.
First, rearrange Equation [1] to make either

or

the subject:


Then we substitute

into Equation [2]






Now we know the value of

which is

, substitute this value into

we have

Answer:
Numbers of books = 13
Numbers of CDs = 7
Answer: 
Step-by-step explanation:
Given
Coordinates of Point A is
i.e. Point A is 3 units to the left and 4 units below x-axis
From figure, we can write

Answer:
48 students in science
Step-by-step explanation:
Let r = students in robots club
s = students in science club
r+s = 84
s = r+12
Substitute the second equation into the first
r+(r+12) = 84
Combine like terms
2r+12 = 84
Subtract 12 from each side
2r = 84-12
2r = 72
Divide by 2
r = 72/2 = 36
We need to find s
s = r+12
s = 36+12
s = 48
There are 48 students in the science club