Will this assumption allow individual bitcoin tracking possible? Fist bitcoins in, First out.

Will this assumption allow individual bitcoin tracking possible?

example,

Addresses A, B and C each has 50 BTC

Transactions, 

  tx1 - A &gt; D - 10 BTC (D - 10 BTC)
  tx2 - B &gt; D - 15 BTC (D - 25 BTC)
  tx3 - C &gt; D - 20 BTC (D - 45 BTC)

  tx4 - D &gt; E - 20 BTC (E - 20 BTC, D - 25 BTC)

  Now,
    D has none of BTC received from A, 5 BTC from B and 20 BTC from C
    E, fist 10 BTC was received from A, next 10 BTC was received from B
This is only true if one does not mix transactions. Otherwise you will have coins that are for example 50% from source A, 10% from B, 40% from C and so forth.