Solving systems using substitutions

Question

Jenny put 6 more marshmallows in her cup than Kristen. Together they had a total of 36 marshmallows. How many marshmallows did they have in each cup of hot chocolate

Answer 1

j = 6+k

j +k = 36

6+k+k = 36

2k = 30

k = 15 marshmellows in Kristen's hot chocolate

j = 15+6 = 21 marshmellows in Jennys hot chocolate