The number of four-digit number greater than or equal to 4321 that can be formed from the digit 0,1,2,3,4,5 allowing for repetition of digits is
If the number is in 5 thousands, there are 6x6x6 ways of choosing it.
If the number is in 4 thousands 4 hundreds there are 2x6x6 ways of choosing it.
If the number is in 4 thousand 3 hundreds, there are 3 x 6 + 5 ways of choosing it.
So, total ways = 6x6x6 + 2x6x6 + 3x6 + 5 = 311
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