Question 705009

Let $b$ be the number of boys and $g$ be the number of girls.

Given $b + g = 70$, so $g = 70 - b$.

The number of students with spectacles is $0.4b + 0.2g = 23$.

Substituting $g$: $0.4b + 0.2(70 - b) = 23$ $\Rightarrow 0.4b + 14 - 0.2b = 23$ $\Rightarrow 0.2b = 9$ $\Rightarrow b = \frac{9}{0.2} = 45$.

Therefore, the number of boys is 45.