Answer:
Therefore, the unknown number is 20.
Explanation:
To find the unknown number, we need to solve the equation.
Let's call the unknown number "x."
According to the problem, the sum of 19 and 21 added to this number equals 40.
So, we can write the equation as:
19 + 21 + x = 40
To solve for x, we need to isolate it on one side of the equation. We can do this by subtracting 19 and 21 from both sides of the equation:
19 + 21 + x - 19 - 21 = 40 - 19 - 21
This simplifies to:
x = 40 - 19 - 21
Calculating the right side:
x = 40 - 19 - 21
x = 20
Therefore, the unknown number is 20.