This question is a great example of what “in-depth understanding” means. (Hint: you don’t even need to use any equations to solve this)

Hint: “Try to be more careful” doesn’t work.

It is quite easy for lower secondary students can do well in exams just by memorising equations and substituting the correct numbers into the equation.

The problem arises when students mistakenly think that this is what is means to "apply concept", when in fact they are just memorising equations

Practice makes perfect