Elimination method
Given :-
$\rightarrow$ for $80$ units it takes $100$ cycles
$\rightarrow$ for $40$ units(load is halved) it takes $10000$ cycles.
$\rightarrow$ for $5000$ cycles$\rightarrow$ __ units ?
My Approach :-
$\rightarrow$ Since we have to calculate for $5000$ cycles the answer would lie between $40$ and $80$.
$\rightarrow$ so, option a and d are eliminated.
$\rightarrow$ now if we see the mid values i.e.
$\rightarrow$ for $40$ and $80$ it would be $60$ and for $100$ and $10000$ it would be $1000$.
$\rightarrow$ so we can eliminate option c.
$\rightarrow$ the only option left is b which is the required answer.