The number of girls in the classroom is 20 * 40% = <<20*40*.01=8>>8 girls
Before new boys join, the number of boys in the classroom is 20 - 8 = <<20-8=12>>12 boys
If 5 new boys join, the new number of boys is 12+5=17 boys and the new total is 20+5=25 total people
The new percentage of girls in the class is 8 girls / 25 total people = 32%
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