Anirbid, Chandranath, Koushik, and Suranjan participated in a puzzle solving competition. The competition comprised 10 puzzles that had to be solved in the same sequence, i.e., a competitor got access to a puzzle as soon as they solved the previous puzzle. Some of the puzzles were visual puzzles and the others were number-based puzzles. The winner of the competition was the one who solved all puzzles in the least time. The following charts describe their progress in the competition. The chart on the left shows the number of puzzles solved by each competitor at a given time (in minutes) after the start of the competition. The chart on the right shows the number of visual puzzles solved by each competitor at a given time (in minutes) after the start of the competition.

Who had solved the largest number of puzzles by the 20-th minute from the start of the competition?

The correct answer is Koushik.

To determine who solved the largest number of puzzles by the 20-th minute, we must examine the chart on the left, titled "All puzzles". This step-chart displays the cumulative number of puzzles solved (on the vertical y-axis) over time in minutes (on the horizontal x-axis) for each of the four competitors.

We locate the 20-minute mark on the x-axis and look vertically at the height of each competitor's line:
• Koushik (represented by the green line): Koushik's line is at a height of
$7$
puzzles solved.
• Chandranath (represented by the yellow line): Chandranath's line is at a height of
$5$
puzzles solved.
• Anirbid (represented by the red line): Anirbid's line is at a height of
$4$
puzzles solved.
• Suranjan (represented by the blue line): Suranjan's line is at a height of
$4$
puzzles solved.

Comparing these values, Koushik had solved the highest number of puzzles, which is
$7$
puzzles, by the 20-th minute of the competition.