Thanks a lot for your reply.

However, this issue has nothing to do with the Data type since I have tried
to change all the possible data types listed. It could be best if you
yourself try this calculation in Infopath, you yourself should see it. I
believe there are something wrong with some particular calculations as
follows:

(1) 2.49   *  4500   =  11,205.000000000 and
(2) 2.47   *  4500   =  11,115

(3) 2.49   *  3000   =  7,470.000000000   and
(4) 2.49   *  1000   =  2490

As you see in the (1) calculation, can you please explain me why how come
this could give a result with ".000000000" while the (2) gives no decimal?
Hope you see my point. The same thing happens for the (3) and (4). Please
also be noted these figures: 2.494, 3.49, 2.499, 2.99 also generate result
with nine decimals when multiply with 4500 or 5000 or 7000 or 9000...And they
give no decimal when multiply with 4900, ...??? It must not be because of the
data type of the field, must it ?
Just to give you more background: all the three fields here are formated as:
Data type: Decimal (double)
Decimal format: Number
                      Decimal places: Auto
                      Use a digit grouping symbol: checked
......

Please kindly have a look at this and reply to us at your earliest
convenience.

Many thanks !