To determine the number of girls in the school, follow these steps:

1. Calculate the number of boys:

   • 70% of the pupils are boys.
   • Number of boys = 70% of 1650 $\text{Number of boys} = 0.70 \times 1650 = 1155$

2. Calculate the number of girls:

   • Total number of students = 1650
   • Number of girls = Total number of students - Number of boys $\text{Number of girls} = 1650 - 1155 = 495$

Thus, there are 495 girls in the school.

Would you like more details or have any questions? Here are some additional questions you might find interesting:

1. What percentage of the students are girls?
2. If 10% of the boys left the school, how many boys would remain?
3. If the number of students increased by 10% next year, what would be the new total?
4. How many more boys than girls are there in the school?
5. If another school with 800 students has 40% girls, how many girls are in that school?

Tip: When dealing with percentages, always convert the percentage to a decimal before performing calculations by dividing by 100.