How many ways can the number $10800$ be expressed as a product of two factors

combinatorics

How many ways can the number $10800$ be expressed as a product of two factors?

My attempt:

$10800 = 2^4 \times 3^3 \times 5^2$

So the number of ways is

$4\times 3\times 2 +1 = 25 $

Is this correct?

Best Answer

As you observed, $10800 = 2^4 \cdot 3^3 \cdot 5^2$.

Thus, each factor has the form $2^a3^b5^c$, where $0 \leq a \leq 4$, $0 \leq b \leq 3$, and $0 \leq c \leq 2$. If the two factors are $2^{a_1}3^{b_1}5^{c_1}$ and $2^{a_2}3^{b_2}5^{c_2}$, then \begin{align*} a_1 + a_2 & = 4 \tag{1}\\ b_1 + b_2 & = 3 \tag{2}\\ c_1 + c_2 & = 2 \tag{3} \end{align*} Equation 1 is an equation in the nonnegative integers with $5$ solutions, namely $(4, 0), (3, 1), (2, 2), (1, 3), (0, 4)$. Similarly, equation 2 is an equation in the nonnegative integers with $4$ solutions, and equation 3 is an equation in the nonnegative integers with $3$ solutions. Thus, there are $$5 \cdot 4 \cdot 3 = 60$$ ordered pairs of factors. Since $10,800$ is not a perfect square, each pair contains two distinct factors. Thus, we have counted each unordered pair of factors twice, so there are $$\frac{5 \cdot 4 \cdot 3}{2} = 30$$ unordered pairs of factors of $10,800$.