My spreadsheet is as follow:
First row:
a1 is sun
a2 is mon
a3 is tue
and so on etc

second row is numbers:
b1 is 1.4
b2 is 13
and so on
My question is how can I sum the second row for the numbers which have cell in first row is sun only.
Please help me. I need your help. Many thanks

For this example i would use a SUMIF

something like this

=SUMIF(A1:A3,"sun",B1:B3)
have a look at the help file for more information

SUMIF works great for one condition. If you have multiple conditions, use SUMPRODUCT

=SUMPRODUCT(($A$1:$A$10="sun")*($B$1:$B$10="East") *($C$1:$C$10="First Qtr"),$D$1:$D$10)

This will look at only those on sun in column A, which also has only items for East in column B, and First Qtr for column C, then sums up those values in column D.

For the sake of total accuracy you should use -- as well in your sumproduct

i.e.

=SUMPRODUCT(--(A1:A3="sun"))
will work for a count but
=SUMPRODUCT((A1:A3="sun"))
wont

this is only needed for single arrays rather than in the above example
as Boolean multiplication does lead to integers

Good point, but that is also why I don't use SUMPRODUCT on single arrays.

I cant believe im making comments on the man who i leant to use this function from in the first place,
i use -- just to make sure im not going to get any errors, and i thought i should point it out, in case cuongvt asks why the SUMPRODUCT function isnt working

In my old age, I tend to get sloppy! Thanks for the reminders.