下面的饼图显示了某天售出的门票数量。当天售出的门票总数为 860 张。求当天售出的学生票数量。

1. Calculate the number of tickets sold to Singaporeans using the right-angle sector.

The sector for Singaporeans is $90^\circ$, which is $\frac{90}{360} = \frac{1}{4}$ of the pie chart.
Number of Singaporean tickets = $\frac{1}{4} \times 860 = 215$

2. Subtract the known ticket numbers from the total to find the number of student tickets.

Number of student tickets = $860 - 172 - 353 - 215$
$= 860 - 740$
$= 120$