Do an atom inventory (list the number of atoms) for each element found in this balanced equation:
3P4 + 10KClO3 ---> 10KCl + 6P2O5

Here's the chart for it:
Reactants-number of atoms, Products-number of atoms
P: 3 X 4 = 12 P:
K: K:
Cl: Cl: 10
O: O: 6 X 5 = 30

In this reactions we have see how many atoms of each element are on left and right side
element          Left side               Right side
P                         3*4                 6*2
Cl                          10*1              10*1
O                         10*3                 6*5
K                       10*1              10*1
The aim of this question is to make you understand that during the reactions atoms are neither created nor be destroyed .