Pentru 3 batoane de ciocolata si 5 pachete de biscuiti, radu a cheltuit 10,50 lei. pentru doua batoane de ciocolata si 3 pachete de biscuiti de acelasi fel, mara a platit 6,55 lei. cate batoane de ciocolata se pot cumpara?

For this case, the first thing we must do is define variables:
 x: price for each bar of chocolate.
 y: price for each package of cookies.
 We write the system of equations:
 3x + 5y = 10.50
 2x + 3y = 6.55
 Solving the system we have:
 x = 1.25 lei
 y = 1.35 lei
 
 Each chocolate bar costs 1.25 lei

AP:PB compares parts to parts. AP:AB compares parts to a whole, this ratio equals K.