Simultaneous equations

problem
Building tetrahedra

Age

14 to 16

Challenge level

Can you make a tetrahedron whose faces all have the same perimeter?
$Overturning fracsum$

problem
Overturning fracsum

Age

16 to 18

Challenge level

Can you solve the system of equations to find the values of x, y and z?
problem
Real(ly) numbers

Age

16 to 18

Challenge level

If x, y and z are real numbers such that: x + y + z = 5 and xy + yz + zx = 3. What is the largest value that any of the numbers can have?
article
A method of defining coefficients in the equations of chemical reactions

A simple method of defining the coefficients in the equations of chemical reactions with the help of a system of linear algebraic equations.
problem
The simple life

Age

11 to 14

Challenge level

The answer is $5x+8y$... What was the question?