Question 175614
Call the number of nickels n.  Then the number of dimes is 7n.  The value of the amount of nickels is 0.05n because each nickel is worth $0.05.  Thus the value of the amount of dimes is 0.10(7n) for the same reason.
So
{{{0.05n + 0.10(7n) = 3.00}}}
{{{0.05n + 0.70n = 3.00}}}
{{{0.75n = 3.00}}}
{{{n = 4}}}
Thus Sally has 4 nickels and 28 dimes.