Question

How may shells does Joshua have?

Answer 1

Joshua has 12 shells

k+ j=54
k +(k-30)=54
k+k=84
2k=84
k=42

j=k-30
j=42-30
j=12


double check 
12+42= 54!!

Answer 2

24, since 54-30 is 24, and 30+24 is 54.