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A school has less than 50005000 students and if the students are divided equally into teams of either 99 or 1010 or 1212 or 2525 each, exactly 44 are always left out. However, if they are divided into teams of 1111 each, no one is left out. The maximum number of teams of 1212 each that can be formed out of the students in the school is

Entered answer:

Solution

✅ Correct Answer: 150

Let's call the number of students NN. From the given conditions:

When divided by 9, 10, 12, or 25: remainder is 4

When divided by 11: remainder is 0 (no one left out)

N<5000N < 5000


When NN is divided by 9, 10, 12, or 25, the remainder is always 4. This means that if the actual number was 4 less, it would be perfectly divisible by 9, 10, 12, and 25

(N−4)(N - 4) is divisible by all of 9, 10, 12, and 25.


Let's find the smallest number that divides 9, 10, 12, and 25.

Prime factorization:

9=329 = 3^2

10=2×510 = 2 \times 5

12=22×312 = 2^2 \times 3

25=5225 = 5^2

Taking the highest power of each prime factor:

LCM(9,10,12,25)=22×32×52\text{LCM}(9, 10, 12, 25) = 2^2 \times 3^2 \times 5^2

=4×9×25=900= 4 \times 9 \times 25 = 900


So N−4=900pN - 4 = 900p for some integer pp.

Therefore: N=900p+4N = 900p + 4

Now, we need to find a p such that N is divisible by 11 & N < 5000

Since, we know N < 5000, p would be < 5.

As maximum value of p is small, we can find the exact value by trial and error.


p=1p = 1

N=900(1)+4 N = 900 (1) + 4

N=904 N = 904

904 is not divisible by 11

p=2p = 2

N=900(2)+4 N = 900 (2) + 4

N=1804N = 1804

1804 is divisible by 11

Since we need to find the maximum value, we should check with the remaining too.

When p=3p = 3, N=2704N = 2704 (Not divisible)

When p=4p = 4, N=3604N = 3604 (Not divisible)

When p=5p = 5, N=4504N = 4504 (Not divisible)


Max number of teams of 12 that can be formed:

N12=150.33\dfrac{N}{12} = 150.33

Hence, the answer is 150.

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