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Find the number of positive integers $$π$$ less than or equal to 100 such that $$π$$ 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 $$0 \le b \le 4$$.
$$n = 2^{a}$$ 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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