Solve the system of equations
We can start by trying to eliminate one of the variables. We notice that the second and third equations have the same coefficient for y (0.4 and 0.3). So, we'll multiply the second equation by 0.75 and subtract the third equation from the resulting equation:
\((0.4x + 0.4y + 0.3z) \times 0.75 = (0.3x + 0.3y + 0.3z)\)
\(0.1x = 5190\)
So, \(x = 51,900\)
Now, we can substitute the value of x in the second equation to find the value of y:
\(0.4(51,900) + 0.4y + 0.3z = 29,400\)
\(20,760 + 0.4y + 0.3z = 29,400\)
\(0.4y + 0.3z = 8,640\)
\(4y + 3z = 43,200\)
We can substitute the value of x in the first equation to find an equation with only y and z:
\(0.6(51,900) + 0.3y + 0.5z = 41,400\)
\(31,140 + 0.3y + 0.5z = 41,400\)
\(0.3y + 0.5z = 10,260\)
\(6y + 10z = 51,300\)
Now, we can solve the new system of equations:
\(4y + 3z = 43,200\)
\(6y + 10z = 51,300\)
We can eliminate y by multiply the first equation by 1.5 and subtracting the second equation:
\(1.5(4y + 3z) - (6y + 10z) = 1.5(43,200) - 51,300\)
\(6y + 4.5z - 6y - 10z = -27\,300\)
\(5.5z = -27\,300\)
So, \(z =-4950\)
However, the negative number of boxes is an infeasible solution to this problem, meaning that there is an error or oversight in the given problem, making it impossible to use up all the stocks with the mentioned fraction of candies for each variety of gift box.
It is important to check the given data for any inconsistencies, as using a negative number of boxes doesn't make sense in this context.