Explanation

1. Number of boys who passed:

$$32\mathrm{\%}\text{ of }450=\frac{32}{100}\times 450=144$$

A number of boys who failed:

$$450-144=306$$

2. Several girls who passed:

$$38\mathrm{\%}\text{ of }550=\frac{38}{100}\times 550=209$$

A number of girls who failed:

$$550 - 209 = 341$$

3. Total students who failed:

$$306 text{ (boys) } + 341 text{ (girls) } = 647$$