The cost of 4 pens and 4 pencil boxes is $100. Three times the cost of a pen is $15 more than the cost of a pencil box. The cost of a pen and a pencil box respectively are

The correct answer is: $10 and $15

Here we have to find that the cost of Pen and pencil,
Firstly , it is given at the cost of 4 pens and 4 pencil is $100 .
Lets pen be x and pencil be y respectively,
So , 4x + 4y = 100
Dividing by 4 both sides,
x + y = 25        ----(1)
And also 3 times cost of pen is $15 more than cost of a pencil box, so we can write,
3x = 15 + y
3x – y = 15      ----(2)
Adding eq(1) and eq(2), we get…
x + y = 25
+ 3x – y =15
----------------------
  4x = 40
   x = 10
substitute x = 10 in eq( 1)
10 + y = 25
y = 25 – 10
y = 15
Therefore , The cost of Pen and pencil $10 and $15.
Or simply,
Let the cost of a pen and a pencil be x and y respectively.
4x+4y=100
x+y=25    ...(1)
3x=y+15
3x−y=15    ...(2)
Adding (1) and (2), we get
4x=40
x=10
Substitute x=10 in equation (1) to get y=15

In this question, we have to make equation on the given date about ages. For making equations , considers the variable of given data and make more than 1 equation to solve.