Assembling a computer

If an operation can be performed in $n_1$ ways, and if for each of these a second operation can be performed in $n_2$ ways, and for each of the first two a third operation can be performed in $n_3$ ways, and so forth, then the sequence of $k$ operations can be performed in $n_1n_2 ... n_k$ ways.

Since $n_1 = 2, n_2 = 4, n_3 = 3$ , and $n_4 = 5$ , there are $n_l\times n_2\times n_3 \times n_4 = 2\times 4 \times 3 \times 5 = 120$ different ways to order the parts.