Let the amount of shells collected by Aaron be 
and the amount of shells collected by Maria be 
respectively. Then the following system of equations should be solved:
First of all, express out of first equation:
Then, replace in second equation with found expression and solve it:
Finally, replace in first equation with found value and solve it too:
So, Aaron has found 7 shells while Maria has found 21.
First of all, express
Then, replace
Finally, replace
So, Aaron has found 7 shells while Maria has found 21.