Actually, I think you will need to start at the top end, with the
largest number to make this work. An approach might be:

1. Find the largest number in the range which is still lower than the
target figure (100)
2. Record the result
3. Sutract the largest number at step 1 from the target figure (120-100
= 20)
4. Repeat from step 1 using the result of step 3 as the new target
figure

For step 1, the MAX worksheet function would provide the basis for
comparison.