If 70% of students in a school are boys and total number of girls in the school is 420, then the number of boys in the school is

Total number of students in school = x (let)

Percentage of girls = 30%

∴ 30% of x = 420

$\Rightarrow x \times \frac{30}{100}=420$

$\Rightarrow x=\frac{420 \times 100}{30}=1400$

$\therefore$ Number of boys $=\frac{1400 \times 70}{100}=980$