Need to get the expected total from the columns in Excel 2007

hi friends...

for example i have a 5 entries in column A1 to A5 (10,20,30,40,51) and in C1 i have 100 (this is the total i am expecting). by manually i know a1:a4 will be 100 but thru macro or formula or any other way is there to find out. suppose if i have 10 or 100 columns.. how can i get this? i need to get the 'X' in B Columns..

for the total 100
i need to get 'X' in the B1, b2,b3,b4

kindly see the attached file.

thanks
sathyguy