**A) 6 days**

B) 10 days

C) 15 days

D) 20 days

One day’s work of x = 1/20

One day’s work of y = 1/12

So work done by x in first 4 days

= 1/20×4= 1/5

So the remaining work be

1-1/5=4/5

now work done by x and y together = 1/20 + 1/12

=2/15

So the remaining work be done by both x and y

= 2/15 ×Days = 4/5

So D = 4/5×15/2 =3×2 = 6

Therefore for 6 days they worked till last.

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