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Andrew Jackson is on the 20 dollar bill.
George Washington is on the one dollar bill and on the quarter.
Abe Lincoln is on the penny and the 5 dollar bill.

Thus:...

Doh!:doh:

I made a stupid. Indeed, Grant is the 18th president. If one substitutes, 16th for 18th, maybe the riddle will make more sense.

I gave too much away on my previous clue it seems.
...

1st,18th,and 7th obviously imply that there is an order to the answers. Perhaps chronological....

Another clue: This riddle may be more difficult for Non-American members...say sorry

I have edited the original to hopefully help out.

As a math equation, it obviously doesnt work. So, It probably should not be approached as such.

I may have been too tricky, but I'll will give some further clues as needed.

Assuming that this is straight algebra would be a mistake. Yes the parentheses imply multiplication, However, substituting for variables would be a mistake. I dont want to give away too much yet.

IF:
20(1ST) = 1(18TH ) = 1(7TH) = 5(1ST)

AND:
100(18TH ) = 1(1ST) = 25(7TH)

THEN:
1 = ?
18 = ?
7 = ?