d= dimes
n = nickels
d =n+7
n*.05+.1*d = 1.90
substitute d = n+7
.05n +.1 (n+7) = 1.9
distribute
.05n +.1n +.7 = 1.90
combine like terms
.15n +.7 = 1.9
subtract .7 from each side
.15n = 1.2
divide by .15
n=8
d=n+7
d=15
you have 8 nickels and 15 dimes