With a set up like this, put the formula below in Sheet2!C2 and drag it down.
VLOOKUP will find the value from Sheet 2 Column A in Sheet 1 Column A and return the value from Sheet 1 Column B.
The formula will then divide the value in Sheet 2 Column B by the value returned by the VLOOKUP and finally use the INT function to return the integer portion so you'll always get a whole number.
1 Part(A) Qty(B)
2 a 5
3 b 12
A B C
1 Part(A1) Qty(B1) Full Units
2 b 6 =INT(B2/VLOOKUP(A2,Sheet1!$A$2:$B$3,2,0))
3 a 12
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