Assuming each number should be used exactly once:
100 10 7 (6-3)/25 = 840
I spent some time "fiddling" with the given numbers and with decomposing 841 before coming up with this answer. Several attempts eventually generated an answer within 1 of the target answer. I could not find a way to get exactly 841, but I cannot prove this is impossible. Nor can I prove that 840 is the closest possible answer - it might be very
possible to use and/or generate decimal amounts so that we can get closer than 1 from the target answer. Thus, I have come close to 841, but I don't know if it is possible to get closer to 841.