Counting might help

a b c d = 210 \large abcd=210

Find the number of integer solutions of the above equation.


If the answer can be expressed in the form 2 n 2^n , enter 2 n 2n .


The answer is 22.

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1 solution

William Allen
Sep 23, 2019

First notice that 210 = 2 3 5 7 210=2\cdot 3\cdot 5\cdot 7

Now let a = 2 x 1 3 y 1 5 z 1 7 ω 1 , b = 2 x 2 3 y 2 5 z 2 7 ω 2 , c = 2 x 3 3 y 3 5 z 3 7 ω 3 , d = 2 x 4 3 y 4 5 z 4 7 ω 4 a=2^{x_1}3^{y_1}5^{z_1}7^{\omega_1} \, , b=2^{x_2}3^{y_2}5^{z_2}7^{\omega_2} \, , c=2^{x_3}3^{y_3}5^{z_3}7^{\omega_3} \, , d=2^{x_4}3^{y_4}5^{z_4}7^{\omega_4}

So a b c d = 2 x 1 3 y 1 5 z 1 7 ω 1 2 x 2 3 y 2 5 z 2 7 ω 2 2 x 3 3 y 3 5 z 3 7 ω 3 2 x 4 3 y 4 5 z 4 7 ω 4 \quad a\cdot b\cdot c\cdot d =2^{x_1}3^{y_1}5^{z_1}7^{\omega_1}\cdot 2^{x_2}3^{y_2}5^{z_2}7^{\omega_2} \cdot 2^{x_3}3^{y_3}5^{z_3}7^{\omega_3} \cdot 2^{x_4}3^{y_4}5^{z_4}7^{\omega_4}

0 x 1 + x 2 + x 3 + x 4 1 0 y 1 + y 2 + y 3 + y 4 1 0 z 1 + z 2 + z 3 + z 4 1 0 ω 1 + ω 2 + ω 3 + ω 4 1 \begin{aligned} \\ 0&\leq x_1+x_2+x_3+x_4&\leq 1 \\ 0&\leq y_1+y_2+y_3+y_4&\leq 1 \\ 0&\leq z_1+z_2+z_3+z_4&\leq 1 \\ 0&\leq \omega_1+\omega_2+\omega_3+\omega_4&\leq 1 \end{aligned}

Each of these equations have 4 4 positive integer solutions meaning a b c d = 210 abcd=210 has 4 × 4 × 4 × 4 = 4 4 4\times 4\times 4\times 4=4^4 positive integer solutions.

Now for all integer solutions we must consider negatives. We can arbitrarily assign the signs to any three positive integers and only the last one will determine if the result is positive or negative. We can assign the signs in 2 3 = 8 2^3=8 ways.

So the number of integer solutions is 2 3 4 4 = 2 3 2 8 = 2 11 2 n = 2 11 = 22 2^3\cdot 4^4=2^3\cdot 2^8=2^{11} \implies 2n=2\cdot 11=\boxed{22}

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