x(.10)+y(.05)=1.50

since each dime we have is another .10 dollars, and each nickel is another .05 dollars, as well as

3x=y

since we have 3 times as many nickels, we have to multiply dimes by 3 to have the same number of coins.

plugging this in to the first equation we get:

x(.10)+3x(.05)=1.50

.1x+.15x=1.5

.25x=1.5

x=6

Sally has 6 dimes.

Plugging this back in to 3x=y, we get

3(6)=y

18=y

So, Sally has 18 nickels