Each question is worth 4 points (100/25=4) Therefore, when you divide 83 by 4, you get 20.75, as as he's aiming to get a B, this needs to be rounded up to 21. Xio would need to get 21 questions right to get a B, hope this helps :)
Including Jamiz, there are 14 girls in the class. If this if half of the total number of pupils (as half are girls, the other half being boys) the total number of students is 14 × 2 = 28 students