Not divisible by 5

There are 1, 2, 3, 0 and 5 numbers with which a five-digit number can be formed. What is the probability of forming numbers that are not divisible by five if repetition of digits is not allowed?


The answer is 0.5625.

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2 solutions

Let P A P_A be the probability that the numbers formed are divisible by 5 5 . Then the probability that the numbers formed are not divisible by 5 5 is 1 P A 1-P_A .

If the last digit is 5 5 , there are 3 × 3 × 2 × 1 × 1 = 18 3 \times 3 \times 2 \times 1 \times 1 = 18 numbers.

If the last digit is 0 0 , there are 4 × 3 × 2 × 1 × 1 = 24 4 \times 3 \times 2 \times 1 \times 1 = 24 numbers.

The total is 18 + 24 = 42 18+24=42 numbers.

The total numbers that can be formed is 4 × 4 × × 3 × 2 × 1 = 96 4 \times 4 \times \times 3 \times 2 \times 1=96 numbers.

So P A = 42 96 = 0.4375 P_A=\dfrac{42}{96}=0.4375 . Finally, 1 P A = 1 0.4375 = 1-P_A=1-0.4375= 0.5625 \color{#D61F06}\boxed{\large 0.5625}

Tasneem Khaled
Jul 23, 2017

We know that a number is divisible by 5 when it has a 0 or a 5 in the unit place. Keeping 5 in the unit place, we arrange the 4 numbers in 4!=24 ways. Numbers that have 0 in the front are meaningless. Keeping 0 in the front, we arrange the 3 numbers in 3!= 6 ways. Numbers that have 5 in the unit place are 24-6=18 numbers. Numbers that have 0 in the unit place can be arranged in 4!=24 ways. Numbers that are divisible by 5 are 24+18=42 numbers. There are 5!=120 numbers that can be formed with all the 5 numbers. Numbers that are meaningless have 0 in the front and they are 4!=24. 120-24=96 numbers. The probability of getting numbers that are divisible by 5 is 42/96= 0.4375 and that of not getting is 0.5625.

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