Unknown number: solving without complications. We are constantly faced with calculations that involve natural numbers, which are addition, subtraction, multiplication and division. Learning to perform operations is very important, but if you are having trouble finding the solution and you don't know how to use operations, it will be of no use. In addition to calculating, therefore, it is important to know how to reason, paying attention to the details provided in the problem to be solved.

Mariana bought 3 pens and a mechanical pencil, and spent 60 reais. If the mechanical pencil cost 24 reais, how much did each pencil cost?

To solve this problem, we need to subtract the total amount (60) from the expense of the mechanical pencil (24).

60-24 = 36.

With that, we have 36 reais was the amount paid in pens. But how do you know how much each one costs? Simply divide 36 by the 3 pens purchased:

36 / 3 = 12

That is, each pen costs 12 reais, while the mechanical pencil costs 24 reais.

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Problems with unknown numbers

Mathematics can be used on a daily basis to solve various problems that revolve around unknown numbers, commonly represented by letters. For example:

What is the number that was added to his double has 12 as a result?

To solve this problem, we must choose the numbers and perform the calculations.


X - unknown number

2x - double the unknown number

x + 2x - unknown number plus double

x + 2x = 12 - problematic equation

To solve, just add the same letters. Check it:

x + 2x = 12

3x = 12

x = 12/3

x = 4

Did you understand Let's see more examples:

unknown number (2)


What is the number that, added to its quadruple, results in the number 25?

X - unknown number

4x - quadruple the number

X + 4x - number added to quadruple

x + 4x = 25 - equation


x + 4x = 25

5x = 25

x = 5/25

x = 5