Time: $$300\div20=15$$ hours.  So, Factory $$B$$ can assemble $$15\times30=450$$ televisions. 
Factory $$A$$ can assemble $$20$$ televisions per hour and Factory $$B$$ can assemble $$30$$ televisions per hour. With these constant rates, if $$A$$ assembles $$300$$ televisions in a period, how many televisions can $$B$$ assemble within the same period? 

[A] $$300$$ 
[B] $$400$$ 
[C] $$450$$ 
[D] $$600$$ 
[E] $$900$$ 