Where do most errors happen in chart questions?

In skipping the units and title. Whether a figure is in thousands or lakhs, a percentage or an amount, decides whether the whole calculation is right.

Should I add an explanation to the answer?

One sentence of interpretation raises the mark, because in banking the meaning of the number matters more than the number.

Where should I practise this from?

From newspaper economic reporting and tables in official publications. Take a table, set yourself three questions and answer them.

The Analytical Section of the Bank Written Exam

বাংলায় পড়তে চাও? দেখো: ব্যাংক লিখিত পরীক্ষার বিশ্লেষণী অংশ

The analytical section is where the bank written paper comes closest to the actual job. An officer reads numbers and reaches a conclusion every day, and this is precisely the capacity being tested.

What it contains

Extracting figures from a table or chart and computing.
Percentage change and identifying a trend.
Comparative analysis — which is larger, and why.
Reasoning questions — a conclusion from given information.
A short comment — what the number is saying.

The order for reading a chart

Read the title and units first — most errors start here.
See what each axis represents.
Mark the largest and smallest values.
Read the question again, then start calculating.

Adding a comment to the answer

Candidates who write the figure and stop score moderately. Those who add a sentence of interpretation — what this rise indicates, which way the trend is going — do better, because in banking it is not the number but its meaning that gets used.

Where to practise from

The best practice material here is not a guide but the economic reporting in newspapers and the tables published by official agencies. Take a table, set yourself three questions on it and answer them — within weeks that habit measurably speeds up how fast you read data.