An integer is considered 'unique' if all of its digits are different. For instance, 58 is 'unique' while 22 is not. How many 'unique' two-digit numbers exist?

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Official Explanation

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To determine the number of 'unique' two-digit numbers (where all digits are distinct), follow these steps:

Tens Digit: Choose from 1 to 9 (9 options, as leading zero is invalid).

Ones Digit: Choose from 0 to 9, excluding the tens digit (9 remaining options).…

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Comments

LightingJan 15, 2025

Great question!

YaminaMar 31, 2025

thanksss

MahbubaApr 12, 2025

can you explain more?

ChiragApr 17, 2025

how much time should this take to solve?

PrashantFeb 15, 2025

This is definitely a test-day trap

RosanneMar 1, 2024

thanks for taking time to upload it!

What this question tests

What it tests

Your ability to manipulate linear and quadratic expressions, substitute unknowns, and solve equations under time pressure.

Common trap

Losing a sign when moving terms across the equals sign, or dividing both sides by a variable that could equal zero.