Jared has 40 pets. If he he has twice as much geckos as toads, and the remaining tenth are snails

Let's say 

 

G = geckos

T = toads

S = snails.

G + T + S = 40

 

We know that a tenth of the 40 pets is snails so

S = 40/10 = 4

 

And we know that

 

G = 2T

Substitute this into G + T + S = 40:

 

2T + T + 4 = 40

3T = 36

T = 12

G = 2T = 24

 

Jared has 24 geckos, 12 toads and 4 snails.