15 - 30 - 15 indicates the percentages of Nitrogen - Phosphorus - Potassium respectively. Thus, the fertilizer has 30% Phosphorus (by mass)
Mass of phosphorous = 5 * 0.3
= 1.5 grams
Atomic mass of Phosphorous:
31
moles = mass / Ar
moles = 1.5 / 31
moles = 0.048 mol
