下面的条形图显示了一组学生在一次数学测试中的成绩分布范围。 在测试中获得至少 45 分的学生占学生总数的几分之几？用最简分数表示你的答案。

1. Read the number of students in each mark range from the bar graph and find the total number of students

Number of students in each range:

• $0-44$: $10$
• $45-74$: $25$
• $75-84$: $45$
• $85-100$: $20$

Total number of students $= 10 + 25 + 45 + 20 = 100$

2. Find the number of students who obtained at least 45 marks

Students with at least $45$ marks are those in the ranges $45-74$, $75-84$, and $85-100$.
Number of students $= 25 + 45 + 20 = 90$
Alternatively, using the total: $100 - 10 = 90$

3. Express the number of students as a fraction of the total and simplify

Fraction $= \frac{90}{100} = \frac{9}{10}$