Pick a variable for the number of quarters, and another variable for the number of half-dollars; say, "q" and "h".
If you have one quarter, how many cents is that? If you have two, how many cents is that? What about three? Four? How about if you have "q" quarters?
Follow this reasoning to find an expression for the value represented by q quarters, and a similar expression for the value represented by h half-dollars.
If you have one quarter, how many of the sixteen coins are half-dollars? What if you have two quarters? Three? Four? How about if you have "q" quarters?
Follow this reasoning to express "h" in terms of "q". Then plug this expression in for "h" in your "value of the half-dollars" expression. This gives you expressions for the values of each coins, all in terms of the one variable.
Sum these two expressions, and set equal to "625", the total value (in cents). Solve.
Eliz.