In fact, for purposes of computing your capital gains, it's assumed that the 100 units you're selling were bought at some kind of average purchase price, or, to put it differently, at the Adjusted Cost Base (ACB).

What's the ACB? It's the ratio: Cost of Purchases/Total Units Held.
Total Units Held ... that's easy to determine, but what's the Cost of Purchases?

The initial Cost of Purchases is just the initial out-of-pocket monies it cost to make your first investment, including any commissions. Let's call this C(1)
The initial Total Units Held we'll call U(1) ... that's what you bought with your $C(1)
Initially, then, ACB(1) = C(1)/U(1) the Magic Ratio

Suppose you later buy more units, say u(2) units.
Then the Total Units Held, after this 2^nd transaction, is:
U(2) = U(1) + u(2)     adding the newly purchased units to the Total Units
and the Cost of Purchases is:
C(2) = C(1) + c(2)    adding the cost of the newly purchased units to the Total Cost
and, as usual, the ratio gives:
ACB(2) = C(2)/U(2).

Note 1: Reinvesting dividends/distributions counts as new purchases!

After n such purchases we have:
C(n) = Σc(i) adding all the purchase costs (including commissions)
U(n) = Σu(i) adding all the units purchased
and, finally, the Magic Ratio:
ACB(n) = C(n)/U(n)

Now comes the interesting part; you sell some units.
Because the ACB is the Adjusted COST Base, it involves the COST of purchasing units and doesn't change when you sell units!
That means that the Magic Ratio must not change after a sale of units:
If the Cost of Purchases and Total Units Held change (by virtue of selling u units)
from C & U    to    C' & U', then
the Magic Ratio = ACB = C'/U' = C/U ... cuz it don't hardly change after a sale

We know what the Total Units Held becomes after this sale of u units,
it's U' = U - u subtracting the u units that were sold

And the Cost of Purchases? What becomes of it?
Since C'/U' = C/U we get
C' = U' C/U and, since U' = U - u
C' = (U - u) C/U = C - u {C/U}
but {C/U} is the Magic Ratio or ACB, so we have, after a sale of u units:
Cost of Purchases = C - u {ACB}

Conclusion? The Cost of Purchases is decreased, after a sale of u units, as though these units were purchased at a cost equal to the current Adjusted Cost Base. See? I told you!

In fact, if you sell 10% of your units then the number of units gets reduced by 10% and so does the Cost of Purchases ... so their ratio remains unchanged! To see this, we write:

U' = U - u = U(1-u/U)

which says the Total Units Held has been reduced by a factor (1-u/U) (because of the sale of u units). In order that the ACB remain unchanged the Cost of Purchases must be reduced by the same factor. That gives:
C'/U' = C (1-u/U)/U (1-u/U) = C/U

Note 2:
Note that, when selling units, your Cost of Purchases is NOT reduced by the money obtained by the sale. It's reduced by a percentage ... the same percentage as your units were reduced. Had you calculated "Cost" by simply adding $$ investments when you buy and subtracting $$ withdrawals when you sell, you'd get something different. (See Note 3, below.)

Because both C and U get reduced by some positive factor when selling, neither can become negative, so ACB = C/U is always a positive number (unless you sell more units than you own).