A can do a work in 21 days. B is 40% more efficient than A. The number of days required for B to finish the same work alone is

I am not sure, may be 15 days is correct

since A takes more time than B .

ratio of time taken by them

$= 140 : 100 = 7 : 5$

let constant ratio $= x$

time taken by $A = 7x$ days

and time taken by $B = 5x$ days

since, A takes $= 21$ days

$7x = 21 => x = 3$

B will finish the work in $= 5x = 5 × 3 = 15$ days

Answer: $15 days$