Ответ:

M(K3PO4) = 39*3 + 31 + 64 = 212

n(K3PO4) = 20 / 212 = 0.09433

N = Na * n

N = 6.02 * 10^23 * 0.09433 = 0.5679 * 10^23 = 5.679 * 10^22