Assuming
#workers in A2: 4,
time taken in B2: 1:30 (time format),
total cases in C2: 1023,
Place
In D2: =A2*(B2*24)
In E2: =D2/C2
Format D2 and E2 as number to taste
D2 returns the total manhours (= #workers x time taken*)
*with the time in B2 converted to decimal time via the "x 24" bit
E2 returns the required manhours per case (= total manhours / total cases)
You could select D2:E2 and simply copy down to return corresponding results
for other sets of inputs in A3:C3. A4:C4, etc