Read the row of digits below from left to right.
3 6 1 2 5 8 4 2 4 8 7 1 3 9 6 3 2 4 1 9 4 8 5 6 2 1
Take every two neighbouring digits of the row as a pair, read from left to right (1st and 2nd, 2nd and 3rd, and so on). In how many of these pairs is the second digit exactly twice the first?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 6
The pairs are read from left to right: for each digit, the digit just after it must be exactly twice it. The order matters, so a pair such as 4 2 (the second is half the first) does not count. The pairs that count are:
3, 6 (1st and 2nd digits)
1, 2 (3rd and 4th digits)
2, 4 (8th and 9th digits)
4, 8 (9th and 10th digits)
2, 4 (17th and 18th digits)
4, 8 (21st and 22nd digits)
In 2 4 8 (8th to 10th digits) the 4 belongs to two pairs, 2, 4 and 4, 8, and both are counted. The pairs 8 4, 4 2, 6 3 and 2 1 read the wrong way round, so they do not count.
So the answer is 6.