Filtering a range with two conditions and totalling the filtered set

Question

I have got to :
=SUMIF(LOOKUP(1, 1::K2:K79,  1::L2:L79), "=✓", 1::I2:I79)
The error message is "SUMIF requires that all range arguments are the same size".
My understanding is that :
LOOKUP(1, 1::K2:k79, 1::L2:l79) will return a set of values filtered from L2:L79 in table 1 that correspond with all rows in K2:K79 (in the same table) that contain the numerical value 1.
That set of values is then used as the first argument in a SUMIF statement to be filtered on "=✓" producing a subset whose corresponding rows in I2:I79 will be totalled.
I've given it a good bit of effort now for a couple of hours and hope that someone can help me out of my amateur quandary with a formula that will achieve the same thing, because I am giving up : ie :
Product
Month
Eligible product (✓) or not (x)
Cost
Teddy Bear
1
x
£35.00
High-resolution PC Monitor
1
✓
£600.00
Electric Fan
1
✓
£45.00
Newspaper subscriptions
1
x
£45.00
Snack foods
1
x
£15.00
Telephone line installation
1
✓
£80.00
Office chair
2
✓
£75.00
I need to efficiently calculate the total cost of "Eligible products" bought in Month 1.

Answer 1

Hi b,
LOOKUP returns a single value corresponding to the first match it finds for the search value.
As I understand it, you wnt the SUM of all 'cost' values where the month is "1" AND 'eligible' is checked.
For a SUM depending on more than one condition being TRUE, the function of choice is SUMIFS.
If Row 1 is a Header row, and Row 179 is the last row in your table (or if no rows below that will match both conditions), you can omit the row part of each range address.
I'm confused as to which column contains the amounts to be summed, so I've made these assumptions:
K = month
L = eligible
M = cost
O2: =SUMIFS(M,K,"=1",L,"=√")
(The formula may go in any cell not referenced by the formula. My choice to place it in O2 was arbitrary.)
Regards,
Barry
PS: Apple has provided an excellent guide to writing formulas in Numbers. The iWork Formulas and Functions User Guide is available for download via the Help menu in Numbers. It provides a description of each function with at least one example of how it may be used.
B