If you simply look at the given numbers, it should be pretty clear that 3 tenths is 3 times 1 tenth.
When Mary puts 1/10 kg in each cage, she can do that for 3 cages with 3/10 kg of bird seed.
__
If you let c represent the number of cages, then the amount of bird seed used will be (1/10)c. This will be equal to the amount available when we use it all for filling cages: