Two successive increases of 10%   [an increase of 21%]:

1 − 4/15 − 5/12   [19/60]:

I did:   4/15 + 5/12  (→ 41/60).
Then:   1 - 41/60  (→ 19/60).

Or:   1 - 4/15  (→ 11/15).
Then:   11/15 - 5/12  (→ 19/60).

4/5 − 1/6 ————————— 4/5 + 1/3

4/5 - 1/6 -> 19/30 4/5 + 1/3 -> 17/15 19/30 * 15/17 -> 19/34