store bought a batch of tapes at a price of 16 yuan per 3 discs, and bought double the previous batch of tapes from another at a price of 21 yuan per 4 discs. If you can get 20% return on your investment by selling all 3 plates at the price of K yuan, what is the value of K?
一商店以每3盘16元钱的价格购进一批录音带，又从另一处以每4盘21元的价格购进比前一批加倍的录音带．如果以每3盘K元的价格全部出售可得到所投资的20%的收益，则K值是多少．
<result>19</result>