Answer:

45 persons

Step-by-step explanation:

Let x be the additional number of persons who are employed after 10 days,

Since, 90 persons can dig a well in 40 days.

Also, 90 persons worked for 10 days,

After which (90+x) person completed the work in 20 days.

Thus, we can write,

Hence, 45 more persons should be employed so as to complete the work in another 20 days.