Boxers = b
Spaniels = s
[tex]b=s*4[/tex]
[tex]s+b=30[/tex]
Now you have two equations - one that tells you the number of boxers in relation to spaniels and one telling you the number of each breed in relation to the total number of dogs. Solve using substitution.
[tex]s+(4*s)=30 \\ 4s+s=30 \\ 5s=30 \\ s=6[/tex]
There are 6 spaniels.