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