Question 3

Find the number of positive integers $$𝑛$$ less than or equal to 100 such that $$𝑛$$ is not divisible by any prime number other than $$2$$ or $$3$$.


Correct Answer: 20

A positive integer contains no prime factors other than $$2$$ or $$3$$ precisely when it can be written in the form $$n = 2^{a}\,3^{b}$$ with non-negative integers $$a,b$$.

We must list all such numbers not exceeding $$100$$.

Upper limits for the exponents:
Because $$2^{6}=64 \lt 100 \lt 2^{7}$$, we need only $$0 \le a \le 6$$.
Because $$3^{4}=81 \lt 100 \lt 3^{5}$$, we need only $$0 \le b \le 4$$.

Case 1: $$b=0$$ Β ($$3^{0}=1$$)

$$n = 2^{a}$$ gives $$1,\,2,\,4,\,8,\,16,\,32,\,64$$ β†’ 7 numbers.

Case 2: $$b=1$$ Β ($$3^{1}=3$$)

$$n = 3\cdot2^{a} \le 100$$ Allowed $$a=0\text{ to }5$$ (because $$3\cdot2^{5}=96 \le 100$$). Numbers: $$3,\,6,\,12,\,24,\,48,\,96$$ β†’ 6 numbers.

Case 3: $$b=2$$ Β ($$3^{2}=9$$)

$$n = 9\cdot2^{a} \le 100$$ Allowed $$a=0\text{ to }3$$ (since $$9\cdot2^{4}=144 \gt 100$$). Numbers: $$9,\,18,\,36,\,72$$ β†’ 4 numbers.

Case 4: $$b=3$$ Β ($$3^{3}=27$$)

$$n = 27\cdot2^{a} \le 100$$ Allowed $$a=0,1$$. Numbers: $$27,\,54$$ β†’ 2 numbers.

Case 5: $$b=4$$ Β ($$3^{4}=81$$)

$$n = 81\cdot2^{a} \le 100$$ Only $$a=0$$ works. Number: $$81$$ β†’ 1 number.

Adding all cases: $$7 + 6 + 4 + 2 + 1 = 20$$.

Therefore, the count of positive integers $$n \le 100$$ with no prime divisors other than $$2$$ or $$3$$ is $$\mathbf{20}$$.

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