If sn=Σ what is the value of ?

Ineed in figuring out this problem.

Ineed your ! i was to turn this into pond first thing i gotta do is outline the inside with this plastic how big should i cut the plastic the height: 10 1/2” the length: 40” the width: 12” me !

Ateacher wanted to buy a chair, a bookshelf, two tables and a desk. she spent $900 for all five items and the chair and the desk combined 70% of her total. if the bookshelf cost $50, how much did each of the tables cost?