Age 14 to 16
This problem is a good one with which to illustrate the precision of meaning that mathematics often requires - in this case the precedence of the operations.
Superficially the task provides some practice at interpreting negative indices but more deeply it invites students to find a simplification for the 'stacked' indices.
The real variable each time is the value, of the four, which is selected as base. To explore that students might go on to consider which sets of four values will produce the same 'stacked' value, irrespective of stacking order.