A is 40% more efficient than B. If B alone can complete a work in 42 days, then A alone can complete the same work in how many days?

Let the efficiency of B be 100.
Since A is 40% more efficient than B, the efficiency of A is:

100+40=140
The ratio of efficiency of A to B is:

Efficiency of B
Efficiency of A
​
=
100
140
​
=
5
7
​

Time taken is inversely proportional to efficiency:

Time taken by B
Time taken by A
​
=
Efficiency of A
Efficiency of B
​

42
Time taken by A
​
=
7
5
​

Time taken by A=
7
5
​
×42=5×6=30 days