Let the total Amount paid be Y
Let the total number of all-white brick be X
Since the Amount paid is dependent on the number of all all-white brick, we have
Y = 300X
since we were told the total Amount paid at the end of the contract is
168000 = 300X
300X =168000
X =168000/300
X = 560.
meaning 560 all-white brick were laid down.