What a twisted question actually looks like
Your book asks the plain version. Your exam moves one small thing and suddenly nobody can solve it. Here is that gap, shown step by step.
Here is something almost every aspirant has felt. You spend a week on rotation. By Friday you are solving sums easily. Two months later the same sum appears in a mock test and you do not even recognise it.
Nothing was forgotten, exactly. Something was never learnt.
The chapter name is a big hint
When all 20 sums come from one chapter, the hardest question is already answered for you: which method to use. You are practising the steps. The exam will test the choice.
In 2020, researchers tested this with 787 students across 54 classes. One group practised chapter by chapter. The other practised the same sums, mixed up. On a later test, the mixed group scored 61%. The chapter-by-chapter group scored 38%.
Why almost nobody does it
Because mixing feels bad. You make more mistakes. You feel slower. There is no nice feeling of "chapter finished". Meanwhile the method that feels smooth is quietly teaching you less.
A simple way to start
Learn a topic in one block — mixing is for practice, not first learning. Then build a set with a few sums from this week, a few from last month, and a few from another subject. Do not label them. Working out the type is the exercise.
This is the cheapest change most aspirants can make. It costs no extra hours. It only costs the good feeling of ticking off a chapter.