Since the theater earned $988 for all adult tickets costing $26 each, we know that they sold:
![\frac{\$ \,988}{\$ 26 / \textrm{ticket}} = 38 \, \textrm{tickets}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/ysypwfak37kqg69ciwap14ip5ijshh9r7k.png)
Additionally, since the theater collected $731 for all student tickets costing $17 each, we know they sold:
![\frac{\$ 731}{\$ 17 / \textrm{ticket}} = 43 \,\textrm{tickets}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/kn9sv2fpdppgvt17mmju81yzcqr3suxljn.png)
The number of tickets sold in total is:
![38 \,\textrm{tickets} + 43 \, \textrm{tickets} = \boxed{81 \,\textrm{tickets}}](https://img.qammunity.org/2019/formulas/mathematics/middle-school/k3kma9ao0ijpgl3ng5icjflkmlrpeaod93.png)
Thus, it can be assumed (1 person = 1 ticket), that 81 people attended the performance.