Answer: x = 20
25 + x + 30 + 2x + 65 = 180, so 3x + 120 = 180, 3x = 60, x = 20.
Frequencies: 0–10: 25, 10–20: 20, 20–30: 30, 30–40: 40, 40–50: 65.
Histogram: take marks on the X-axis (1 cm = 10 marks) and number of students on the Y-axis (1 cm = 10 students). Draw adjacent rectangles on each class interval with heights 25, 20, 30, 40 and 65.