To find the total number of students in the classroom given the ratio of boys to girls and the number of boys, let's break down the problem.
The ratio of boys to girls is 4:5. This means for every 4 boys, there are 5 girls.
Since there are 20 boys in the classroom, we can use this ratio to find the number of girls.
The number of boys (20) corresponds to the first part of the ratio (4). To find the scaling factor, we divide the number of boys by the ratio part for boys:
\[ \text{Scaling factor} = \frac{20 \text{ boys}}{4} = 5 \]
Now we can find the number of girls using the scaling factor. The ratio part for girls is 5, so we multiply the scaling factor by 5:
\[ \text{Number of girls} = 5 \times 5 = 25 \]
Now, to find the total number of students in the classroom, we add the number of boys and girls together:
\[ \text{Total students} = \text{Number of boys} + \text{Number of girls} = 20 + 25 = 45 \]
Thus, the total number of students in the classroom is \( \boxed{45} \).