All currency coins or notes are issued in these 4 denominations: {1, 2, 5, 10} but not {3, 4, 6, 7, 8, 9}.
Reason: These 4 numbers {1, 2, 5, 10} just need at most 2 operations (+ or -) to generate any number from 1 to 10.
(1 = 1)
(2 = 2)
3 = 1 + 2
4 = 2 +2
(5 = 5)
6 = 1 + 5
7 = 2 + 5
8 = 10 – 2 = 1 + 2 + 5
9 = 10 – 1 = 2 + 2 + 5
(10 = 10)
