Question: Write out all of the multiplication schemes for four numbers $a_1,a_2,a_3,a_4$ in that order.
My Attempt: By explicating listing the elements we get the answer as $5$.
$$ (((a_1a_2)a_3)a_4) $$ $$ ((a_1a_2)(a_3a_4)) $$ $$ ((a_1(a_2a_3))a_4)$$ $$ (a_1((a_2a_3)a_4))$$ $$ (a_1(a_2(a_3a_4))) $$
Doubt: Is my answer correct? Is there a better way of doing this?