%
% Computing the age of the son.
%
% The age of the son  today :  x
% The age of the mother today: y
% ------------------------------
% The age of the son after 6 years: x+6
% The age of the mother after 6 years: y+6
% ----------------------------------------
%
% equations:
% ----------
% today:         y=x+21;
% after 6 years: y+6   =   5( x+6)
%
% simplifying the equations.
% --------------------------
%
%       y=x+21
%       y+6=5x+30
% 
%       y=x+21
%       y=5x+30-6=5x+24
%
%       x -y = -21
%      5x -y = -24
%
 c=[1 -1];
 d=[5 -1];
 A=[c ; d];
 b=[-21 ; -24];
 
 % inv for matrix inverse
 sol= inv(A)*b; % solution
 sol