Răspuns :
Numerele naturale de 3 cifre sunt cele de la 100 la 999. In total sunt 900 astfel de numere.
{100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 1nr
110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 1nr
120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 1nr
130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 1nr
140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 10 nr
150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 1 nr
160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 1 nr
170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 1 nr
180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 1 nr
190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 1 nr
total 19
200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 1 nr
210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 1 nr
220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 1 nr
230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 1 nr
240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 10 nr
250, 251, 252, 253, 254, 255, 256, 257, 258, 259,, 1 nr
260, 261, 262, 263, 264, 265, 266, 267, 268, 269,, 1 nr
270, 271, 272, 273, 274, 275, 276, 277, 278, 279,, 1 nr
280, 281, 282, 283, 284, 285, 286, 287, 288, 289,, 1 nr
290, 291, 292, 293, 294, 295, 296, 297, 298, 299, , 1 nr
total 19
300, 301, 302, 303, 304, 305, 306, 307, 308, 309, , 1 nr
310, 311, 312, 313, 314, 315, 316, 317, 318, 319,, 1 nr
320, 321, 322, 323, 324, 325, 326, 327, 328, 329,, 1 nr
330, 331, 332, 333, 334, 335, 336, 337, ..., 1 nr
340,341.....................................................................10 nr
350..... 1 nr
360..... 1 nr
370..... 1 nr
380..... 1 nr
390... 1 nr
total 19
400.... 10 nr
410.... 10 nr
420.... 10 nr
430.... 10 nr
440.... 10 nr
450.... 10 nr
460.... 10 nr
470.... 10 nr
480.... 10 nr
490.... 499 10 nr
total 100
500.... 599 total 19
600.... 699 total 19
700.... 799 total 19
800.... 899 total 19
900.... 999 total 19
deci avem 8*19+100=152+100=252 numere care contin cifra 4
900-252=648 numere care nu contin cifra 4
probabilitatea sa nu contina cifra 4 =648/900 =18/25
{100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 1nr
110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 1nr
120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 1nr
130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 1nr
140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 10 nr
150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 1 nr
160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 1 nr
170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 1 nr
180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 1 nr
190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 1 nr
total 19
200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 1 nr
210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 1 nr
220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 1 nr
230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 1 nr
240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 10 nr
250, 251, 252, 253, 254, 255, 256, 257, 258, 259,, 1 nr
260, 261, 262, 263, 264, 265, 266, 267, 268, 269,, 1 nr
270, 271, 272, 273, 274, 275, 276, 277, 278, 279,, 1 nr
280, 281, 282, 283, 284, 285, 286, 287, 288, 289,, 1 nr
290, 291, 292, 293, 294, 295, 296, 297, 298, 299, , 1 nr
total 19
300, 301, 302, 303, 304, 305, 306, 307, 308, 309, , 1 nr
310, 311, 312, 313, 314, 315, 316, 317, 318, 319,, 1 nr
320, 321, 322, 323, 324, 325, 326, 327, 328, 329,, 1 nr
330, 331, 332, 333, 334, 335, 336, 337, ..., 1 nr
340,341.....................................................................10 nr
350..... 1 nr
360..... 1 nr
370..... 1 nr
380..... 1 nr
390... 1 nr
total 19
400.... 10 nr
410.... 10 nr
420.... 10 nr
430.... 10 nr
440.... 10 nr
450.... 10 nr
460.... 10 nr
470.... 10 nr
480.... 10 nr
490.... 499 10 nr
total 100
500.... 599 total 19
600.... 699 total 19
700.... 799 total 19
800.... 899 total 19
900.... 999 total 19
deci avem 8*19+100=152+100=252 numere care contin cifra 4
900-252=648 numere care nu contin cifra 4
probabilitatea sa nu contina cifra 4 =648/900 =18/25
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