The difference of the square of a number and 40 is equal to 3 times that number. Find the positive solution.

The answer is 8.

Step-by-step explanation: The first step is to convert the expression into figures. We shall call the unknown number Y. So if we are told “the square of a number,” that means Y squared, or better still, Y^2. Further we are told “the difference between the square of a number and 40” and that can be written as;

Y^2 - 40.

Next we are told that this expression is equal to 3 times that number (that is 3Y). That can now be written out as follows,

Y^2 - 40 = 3Y

If we move all expressions to one side of the equation, what we would have is,

Y^2 - 3Y -40 = 0

(Remember that when a positive value crosses to the other side of an equation it becomes negative and vice versa)

We now have a quadratic equation

Y^2 -3Y - 40 = 0

By factorizing we now have

(Y -8) (Y + 5) = 0

Therefore Y - 8 = 0 or

Y + 5 = 0

Hence, Y = 8 or Y = -5

Since we are asked to calculate the positive solution, Y = 8