We need to complete the given letter-number series for questions 18, 19, 20, and 21.

1. Problem Description

We need to complete the given letter-number series for questions 18, 19, 20, and

2

1.

2. Solution Steps

* Problem 18: AB50, DE49, GH48, JK47, \_\_\_, \_\_\_

* Letters: The first letters are A, D, G, J. The differences in position in the alphabet are $D-A = 3$, $G-D=3$, $J-G=3$. So the next letters should be $J+3=M$ and $M+3=P$. The second letters are B, E, H, K. The differences are $E-B=3$, $H-E=3$, $K-H=3$. So the next letters should be $K+3=N$ and $N+3=Q$.

* Numbers: The numbers are 50, 49, 48,

4

7. So the next numbers should be 46 and

4

5. * Therefore, the next two terms in the series are MN46 and PQ

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5.

* Problem 19: Z100, Y200, X300, W400, \_\_\_, \_\_\_

* Letters: The letters are Z, Y, X, W. The positions are decreasing by 1 each time. So the next letters should be V and U.

* Numbers: The numbers are 100, 200, 300,

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0

0. The numbers are increasing by 100 each time. So the next numbers should be 500 and

6

0

0. * Therefore, the next two terms in the series are V500 and U

6

0

0.

* Problem 20: PQ46, RS55, TU64, \_\_\_, \_\_\_

* Letters: The first letters are P, R, T. The differences in position in the alphabet are $R-P = 2$, $T-R=2$. So the next letters should be $T+2=V$ and $V+2=X$. The second letters are Q, S, U. The differences are $S-Q=2$, $U-S=2$. So the next letters should be $U+2=W$ and $W+2=Y$.

* Numbers: The numbers are 46, 55,

6

4. The differences are $55-46=9$ and $64-55=9$. So the next numbers should be $64+9=73$ and $73+9=82$.

* Therefore, the next two terms in the series are VW73 and XY

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2.

* Problem 21: H18I, G17H, F16G, \_\_\_

* Letters: The first letters are H, G, F. The letters are decreasing by 1 each time. So the next letter should be E.

* Numbers: The numbers are 18, 17,

1

6. The numbers are decreasing by 1 each time. So the next number should be

1

5. * The third letters are I, H, G. The letters are decreasing by 1 each time. So the next letter should be F.

* Therefore, the next term in the series is E15F.

3. Final Answer

* Problem 18: MN46, PQ45

* Problem 19: V500, U600

* Problem 20: VW73, XY82

* Problem 21: E15F