Two-Digit Numbers?
How many two-digit numbers are not divisible by 4 nor 5?
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answers:
Puzzling say: There are 90 numbers that have two digits (10-99).

1/5 are divisible by 5, so 4/5 are not.
90 * 4/5 = 72

1/4 of these are divisible by 4, so 3/4 are not.
72 * 3/4 = 54

Answer:
54

Specifically:
{11, 13, 14, 17, 18, 19, 21, 22, 23, 26, 27, 29, 31, 33, 34, 37, 38, 39, 41, 42, 43, 46, 47, 49, 51, 53, 54, 57, 58, 59, 61, 62, 63, 66, 67, 69, 71, 73, 74, 77, 78, 79, 81, 82, 83, 86, 87, 89, 91, 93, 94, 97, 98 and 99}
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Johnathan say: Divisible by 4: 12, 16, 24, 28, 32, 36, 44, 48, 52, 56, 64, 68, 72, 76, 84, 88, 92, 96. 18 numbers all told.

Divisible by 5: 10, 15, 25, 30, 35, 45, 50, 55, 65, 70, 75, 85, 90, 95. 14 numbers all told.

Divisible by both 4 and 5: 20, 40, 60, 80. 4 numbers all told.

90 (total 2-digit numbers) - 36 (2-digit numbers divisible by 4, 5, or both)...

= 54 2-digit numbers indivisible by 4 or 5.
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ted s say: 19 + 24 - 4....number of 5' plus number of 4's minus the number of 20's
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