A student places 2.5 grams of salt in 
90 ml. of water. She then transfers 30 ml. 
of that solution to an evaporating dish. 
How many grams of salt are in the 30 ml. 
of solution?

Use the proportion:

2.5 g of salt IS TO 90 ml of liquid AS 
      x g of salt IS TO 30 ml of liquid

Replace the "IS TO"'s by " / "'s and the 
"AS" by " = "

  2.5/90 = x/30

Solve that and get .833333 grams of salt
(or 5/6 grams if you use fraction 2 1/2,
which is 5/2 instead of decimal 2.5)

Edwin