how do you solve 18, 19 20 quickly?

lily88888UID: 2828533

An organization will hold its first six annual meetings in exactly six cities-Los Angeles, Montreal, New York, Toronto, Vancouver, and Washington-using each city only once. The following conditions govern the order in which the cities are used:
Los Angeles must be used in some year after the year in which Toronto is used.
Vancouver must be used either immediately before or immediately after Washington.
The meeting in Toronto must be separated from the meeting in Montreal by meetings in exactly two other cities.
The meeting in Vancouver must be separated from the meeting in Los Angeles by meetings in exactly two other citi .

18. Which one of the following must be true'?
(A) Toronto is used in the first year.
(B) Montreal is used in the fourth year.
(C) Toronto is used at some time before Montreal is used.
(D) New York is used either immediately before or immediately after Vancouver.
(E) The meeting in New York is separated from the mceting in Washington by meetings in exactly two other cities.

19. There is exactly one possible order in which the cities are e if which one of the following is true?
(A) Los Angeles is used in the fifth year.
(B) Montreal is used in the sixth year.
(C) New York is used in the fifth year.
(D) Vancouver is used in the first year.
(E )Washington is used in the second year.

20. Which one of the following is a complete and accurate list of the years in which Washington could be used?
(A) 1 3, 5
(B) 2, 3,4, 5
{C) 2,3,4,6
(D) 1, 2, 4, 6
(E) 1, 2, 3, 4, 5, 6

21. If Montreal is used in the first year, which one of the following CANNOT be true?
(A) Washington is used in the third year.
(B) Vancouver is used in the third year.
( C) Toronto is used in the fourth year.
(D) New York is used in the fifth year.
(E) Los Angeles is used in the third year.

22. Which one of the following could be true'?
(A) Los Angeles is used in the first year.
(B) New York is used in the second year.
(e) Montreal is used in the third year.
(D) Vancouver is used in the fourth year.
(E) Toronto is used in the sixth year.

23.which one of the following must be false'?
( A) Los Angeles is used either immediately bcfore or immediately after New York.
(B) Los Angeles is used either immediately before or immediately after Washington.
(C) New York is used either immediately before or immediately after Toronto.
(D) Toronto is used either immediately before or immediately after Vancouver.
(E ) Toronto is used either immediately before or immediately after Washington.

duijinxiaoziUID: 2585955

画图：
1)Los Angeles must be used in some year after the year in which Toronto is used.
T<L
2)Vancouver must be used either immediately before or immediately after Washington.
VW  or  WV
3)The meeting in Toronto must be separated from the meeting in Montreal by meetings in exactly two other cities.
T _ _ M    or  M _ _T
4)The meeting in Vancouver must be separated from the meeting in Los Angeles by meetings in exactly two other citi
L _ _ V    or  V _ _ L
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因为T先开，然后L开。
所以是 T _ _ M 的顺序。而且 L 在 T 和M 之间。
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第一种情况：
T L N M V W

第二章情况：
T N L M W V

lily88888UID: 2828533

There are 6 more possiblities,
WVTNLM
VWTLNM

TLNMVW
TNLMWV

MVWTLN
MWVTNL

NTVWML
NMVWTL

You gave me the clue that connect T and L. I was focusing on how to put (VW)TL together.

Thank you so much. It might have taken you a lot time.