A school has 200 students, of which 7/10 are boys. How many girls are in the school?

Introduction:
A simple fraction and complement problem: if a fraction of the total are boys, the rest are girls. Convert the fraction to a count and subtract from the total.

Given Data / Assumptions:
Total students = 200. Boys = 7/10 of total. Girls = total − boys.

Concept / Approach:
Compute boys as (7/10) * 200, then subtract from 200 to get girls. This is a direct application of fractions and complements.

Step-by-Step Solution:
Boys = (7/10) * 200 = 140. Girls = 200 − 140 = 60.

Verification / Alternative check:
Girls are 3/10 of the total. (3/10) * 200 = 60, confirming the result.

Why Other Options Are Wrong:
80 and 120: These correspond to 40% and 60% and do not match the remaining 30%. 40: Too small (20%). 140: That is the number of boys, not girls.

Common Pitfalls:
Confusing the fraction for boys with the fraction for girls or miscomputing (7/10) * 200. Always compute one and subtract from the total.

Final Answer:
The number of girls is 60.