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What number comes in the blank in each sequence below? 
1. 4, 8, 16, 32, 64, _______ Answer: 128Each number is multiplied by 2 to get the next number.
2. 2, 3, 5, 7, 11, 13, 17, 19, _______ Answer: 23These are the prime numbers (i.e., numbers whose only factors are one and the number itself).
3. 1, 3, 6, 10, 15, _______ Answer: 213  1 = 2; 6  3 = 3; 10  6 = 4; 15  10 = 5; The differences between two consecutive numbers are 2, 3, 4, 5, 6, ... So, 21  15 = 6. Alternatively, 1 + 2 = 3; 1 + 2 + 3 = 6; 1 + 2 + 3 + 4 = 10; 1 + 2 + 3 + 4 + 5 = 15; 1 + 2 + 3 + 4 + 5 + 6 = 21; The nth term in the sequence is given by n (n + 1)/2. The numbers are often referred to as triangular numbers.
4. 500, 475, 425, 350, 250, _______ Answer: 125500  475 = 25; 475  425 = 50; 425  350 = 75; 350  250 = 100; The differences between two consecutive numbers are 25, 50, 75, 100, 125, ... So, 250  125 = 125.
5. 200, 196, 180, 116, _______ Answer: 140200  196 = 4; 196  180 = 16; 180  116 = 64; The differences between two consecutive numbers are 4, 16, 64, ... (each number starting with 4 is multiplied by 4 to get the next number). So, 116  4(64) = 116  256 = 140.
6. 987, 878, _______, 660, 551, 442 Answer: 769987  109 = 878; 878  109 = 769; 769  109 = 660; 660  109 = 551; 551  109 = 442; The difference between two consecutive numbers is 109.
7. 213, 426, _______, 852, 1065, 1278 Answer: 639213 + 213 = 426; 426 + 213 = 639; 639 + 213 = 852; 852 + 213 = 1065; 1065 + 213 = 1278; The numbers simply increase by 213.
8. 144, 121, 100, _______, 64, 49 Answer: 81The terms are merely the squares of integers starting with 12 in descending order. Thus, 12^{2} = 12 x 12 = 144; 11^{2} = 11 x 11 = 121; 10^{2} = 10 x 10 = 100; 9^{2} = 9 x 9 = 81; 8^{2} = 8 x 8 = 64; 7^{2} = 7 x 7 = 49; Alternatively, the differences between consecutive terms form the following simple sequence: 23, 21, 19, 17, 15.
9. 8, 27, 64, _______, 216, 343 Answer: 125The terms are merely the cubes of integers starting with 2. Thus, 2^{3} = 2 x 2 x 2 = 8; 3^{3} = 3 x 3 x 3 = 27; 4^{3} = 4 x 4 x 4 = 64; 5^{3} = 5 x 5 x 5 = 125; 6^{3} = 6 x 6 x 6 = 216; 7^{3} = 7 x 7 x 7 = 343;
10. 2, 3, 4, 12, 6, 23, 8, 36, 10, _______ Answer: 51The odd terms of the sequence are simply even numbers. The even terms are 3, 12, 23, 36, ... 12  3 = 9; 23  12 = 11; 36  23 = 13; The differences between two consecutive even terms are 9, 11, 13, 15, ... So, 51  36 = 15.
11. 3, 10, 20, 27, _______, 44 Answer: 37The odd terms continually increase by 17, i.e., 3, 20, 37, ... The even terms also continually increase by 17, i.e., 10, 27, 44, ... Alternatively, add 7 and 10 alternately starting with 3.
12. 2, 7, 11, 16, _______, 25 Answer: 20The odd terms continually increase by 9, i.e., 2, 11, 20, ... The even terms also continually increase by 9, i.e., 7, 16, 25, ... Alternatively, add 5 and 4 alternately starting with 2.
13. _______, 0.99, 9.9, 99, 990, 9900 Answer: 0.099Each number is onetenth the number following it. Thus, (1/10) x 0.99 = 0.099
14. 2.5, 3.75, _______, 8.4375, 12.65625 Answer: 5.625Each number is 1.5 times the number preceding it. Thus, 1.5 x 3.75 = 5.625
15. 243, 162, 108, 72, _______, 32 Answer: 48Each number is twothirds the number preceding it. Thus, 72 x 2/3 = 48
16. 1/24, _______, 7/18, 10/15, 13/12, 16/9 Answer: 4/21The numerators (starting with 1) increase continually by 3, and the denominators (starting with 24) decrease continually by 3. The numerators are 1, 4, 7, 10, 13, 16, ... The denominators are 24, 21, 18, 15, 12, 9, ...
17. 2, 6, 14, _______, 62, 126 Answer: 306  2 = 4; 14  6 = 8; 30  14 = 16; 62  30 = 32; 126  62 = 64 The differences between two consecutive numbers are 4, 8, 16, 32, 64, ... (a sequence where each number is multiplied by 2 to get the next number starting with 4).
Try the Quiz : IQ Tests : Number Sequences I
