QUESTION

CSAT

Medium

Maths

Prelims 2022

The digits 1 to 9 are arranged in three rows in such a way that each row contains three digits, and the number formed in the second row is twice the number formed in the first row; and the number formed in the third row is thrice the number formed in the first row. Repetition of digits is not allowed. If only three of the four digits 2, 3, 7 and 9 are allowed to use in the first row, how many such combinations are possible to be arranged in the three rows?

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Explanation

We can only use three of the four digits—2, 3, 7, and 9—in the first row.

Since the third row is a three-digit number and is thrice the first row number, the maximum possible value for the first row is 987/3=329987/3 = 329. This constraint narrows down the possible values of the first row to:

237, 273, 239, 293, 279, 297, 327, and 329.

Now, let’s check each of these values by doubling and tripling them to see if they satisfy the conditions without any digit repetition.

237×2=474237 \times 2 = 474 (digit repetition, so eliminate) 273×2=546273 \times 2 = 546, 273×3=819273 \times 3 = 819 (No digit repetition, so this is a valid combination) 239×2=478239 \times 2 = 478, 239×3=717239 \times 3 = 717 (digit repetition, so eliminate) 293×2=586293 \times 2 = 586, 293×3=879293 \times 3 = 879 (digit repetition, so eliminate) 279×2=558279 \times 2 = 558 (digit repetition, so eliminate) 297×2=594297 \times 2 = 594, 297×3=891297 \times 3 = 891 (digit repetition, so eliminate) 327×2=654327 \times 2 = 654, 327×3=981327 \times 3 = 981 (No digit repetition, so this is a valid combination) 329×2=658329 \times 2 = 658, 329×3=987329 \times 3 = 987 (digit repetition, so eliminate)

After checking all cases, only 273 and 327 satisfy the conditions without digit repetition.

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