I found an error in solution for 5/2/20 Daily Magic Spells. It says,"The numbers that leave a remainder of 10 when we use them to divide 100 are 15, 30, 45 and 90."
18 is missing!
My solution:
100 ≡ 10 (mod n), 100 ≡2 (mod n-1)
⇔n丨90, n-1丨98
n are factors of 90 larger than 10: 15, 18, 30, 45, 90
Among them, the only n that satisfies n-1丨98 is 15
∴n=15