Grade five math: There are snakes, toads (three legs) and frogs (four legs) in the pool. Now there are 3600 heads, 13000 legs. How many?

Suppose a toad has x frogs, y snakes and z snakes?

So * * * has 3x+4y legs, x+y+z = 3600-( 1).

Because 3x+4y= 13000? And x+y is less than 3600.

Combinatorial equation? Suppose x+y=3600.

(2).3x+4y= 13000？ (3).x+y=3600？

(3) Substitute into formula (2)? x= 1400，y=2200

Because animals cannot be half? X must be an integer multiple of 100?

And: 0

So what is the value of x? 100、200、。 .。 . . 1300。 ? Substitute into equation (2)

The values of y are 3 175, 3 100,. . . . . 2275。 X.y substitution (1).

The number of snakes can be taken as 325,300,. . 25。