Sally has $1.50 worth of change in her pocket-- all nickels and dimes. she has three times as many nickels as dimes. find the number of each

If we use x for the number of dimes, and y for the number of nickels, then from this problem we know:
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