Solving linear systems by substitution

Question

Mariko has 30 nickels and dimes. She has 12 more nickels then dimes. How many dimes does mariko have?

Answer 1

N = D+12 <--- 12 more nickels than dimes

N + D = 30

D+12 + D = 30

2D + 12 = 30

2D = 18

D = 9

9 dimes , 21 nickels