23. Kim and Jay leave at the same time to travel 25 miles going to the beach.
Kim drives 9 miles in 12 minutes
Jay drives 10 miles in 15 minutes.
Question: who will arrive first?
Unit rate of Kim
=> 9 miles in 12 minutes
=> 0.75 miles / minutes
=> 25 / 0.75
It will take 33.33 minutes for Kim to arrive
Unit rate of Jay
10 miles in 15 minutes.
=> 10 miles /15 minutes
=> 0.67 miles /minute
=> 25 / 0.67
It will take Jay 37.31 minutes to arrive at the beach.
SO Kim will arrive first
For the first digit of the required 20-digit number, we can have 9 choices for the number, those are the digits 1 to 9. In the second digit of the number, we will only have 8 choices. That is because, we don't want to write a number in the second digit that is similar to the first digit and the third digit.
Similarly, in the third digit's place, we will only have 8 choices because we do not want the digit to be similar to the second digit and the fourth digit. This will go on up until the 19th digit. In the 20th digit, we will also have 9 choices (from 0-9) but we do not want the digit to be similar to the 19th digit.
From the explanation given above, we have the number of ways,
n = 9 x 8^(19 - 1) x 9 = 1.459 x 10^18
<em>ANSWER: 1.459 x 10^18</em>
