IF function to buy inventory if it goes below 75...

i want to buy inventory to the BUY ORDER on B5 ... wen the INVENTORY on B4... value.. that STARTS with 100 on C4 drops under 75 because of PRODUCT SALES on B3... the sales start on D3 using a IF fuction on C5 and i dont even know if the IF function can aply in this ....

thank you

regards

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20 rows of data is enough (probably)

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THANK YOU ALL VERY MUCH im very happy i CRACKED THE CASE !!!!!!!!!! it turns out its actualy VERY simple im not going to post the outcome cause maybe someone is outthere taking the same course and its lame to go and post it.... i was searching for easy way to crack this i must admit!!!!!!!! but i was not realy geting it!!! i only got it wen i read in portuguese writing.... what it is and how a IF function works !!! im portuguese my exel is in portuguese and im taking a course in english and the teacher uses exel english version of course and its has diferences in writing functions commas dont aply on my exel etc theres a lot this difrent for a beginer its messed up im watching videos of brazilians dudes on you tube know to get it rigth on track

im not dumb just a litle retard ahahah it toke me maybe 2 or 3 days to do this function right!!!!

well i was thinking i had the correct if statement but its not the case i could any one look at it and say somthing about it?
regards

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Thanks