Works fine for me in Firebird 2.5.9, 3.0.10 and 4.0.2 with a dialect 3
database.
I can reproduce this with a dialect 1 database, which is not surprising,
because there the number is actually a double precision, and 1.65 is not
exactly 1.65 in double precision, and neither is the result of 51 * 1.65
exactly 84.15, but rather closer to 84.14999999999999, so when rounding,
it is rounded down.
You should stop using dialect 1, it was deprecated back in 2000 with
InterBase 6.
Mark
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Mark Rotteveel