Question 155482
Let x= number of Tom's columns
Let y= number of David's columns

Tom and David have 480 marbles between them
Tom arranges them in columns with 6 in each column, and David arranges them with 9 in each column. 
then 
{{{6x+9y=480}}}
The total number of columns is 72. 
then
{{{x+y=72}}}