How many 3 letter codes are possible using a-j without repeats?

There are 10 letters in the set {a, b, c, d, e, f, g, h, i, j} which is the pool of letters to choose from when making these three letter codes.

We have 10 choices for slot 1
Then 9 choices for slot 2. This is because we can't reuse the choice for slot 1
Then 8 choices for slot 3

Overall, there are 10*9*8 = 90*8 = 720 different permutations

720

Note: you can use the nPr permutation formula with n = 10 and r = 3 to get the same answer

When Riko left the house, Yuto was 5.25 miles ahead of her.

-- From the time she left the house, she gained (0.35 - 0.25) = 0.1 mile on Yuto

every minute ... closing the gap at the rate of 0.1 mile per minute.

-- If she can keep it up, she'll close the gap in (5.25 / 0.1) = 52.5 minutes.

-- Riko is behind her brother when  0 ≤ t < 52.5 minutes .

Step-by-step explanation:mark brainleists