Let A have a people and B have a + 20 people.<br><br>
Total score from A = 70a. Total score from B = 85(a + 20) = 85a + 1700.<br>
Combined total score = 70a + 85a + 1700 = 155a + 1700.<br>
Combined number of people = a + (a + 20) = 2a + 20.<br><br>
Combined average = (155a + 1700) ÷ (2a + 20) = 79.<br>
155a + 1700 = 79(2a + 20) = 158a + 1580.<br>
1700 − 1580 = 158a − 155a.<br>
120 = 3a, so a = 40.<br><br>
A has 40 people, B has 40 + 20 = 60 people.<br>
Check: (40×70 + 60×85) ÷ 100 = (2800+5100)÷100 = 79. ✓