(1)  Since 20% of the cards in the box are red and the number of blue card is 1/4 of the number of red cards,

     it means that the number of red and green cards altogether is 

     0.2 +  = 0.25 = 25% of the total number of cards.



(2)  Since among the remaining 150 cards the number of green cards is 2/3 of the number of white cards,

     for the number of green cards x we have this equation

         x = ,

     which gives

        x = 60.   <<<---===  the number of green cards.



(3)  The remaining 150 cards comprise 100% - 25% = 75% = 0.75 of the total number of cards.

     Hence, the total number of cards is   = 200 cards.



(4)  Finally, the percentage of green cards in the box is  =  = 30%.    ANSWER