Use the numbers $$a_1,a_2,\dots,a_n$$ and the operations $$o_1,o_2,\dots,o_m$$ (for example $$+,\times,!$$) to form the target number $$N$$.
Other restrictions may be added. For instance, it may be specified that each of $$a_1,a_2,\dots,a_n$$ may be used exactly once, or only in a certain order.