Call the weights of the two dogs 
and 
.
We know that Tripp weights 35lbs more than Rico, or:
We also know that the total weight of both dogs is 49lbs.:
Now, by substitution:
Rico weighs 7lbs.
Then:
Tripp weighs 42lbs.
To check our work:
CHECK!
We know that Tripp weights 35lbs more than Rico, or:
We also know that the total weight of both dogs is 49lbs.:
Now, by substitution:
Rico weighs 7lbs.
Then:
Tripp weighs 42lbs.
To check our work: