GIVEN: Saral, Ravi and Karan alone takes 10, 12 and 15 days, respectively to complete a task. FORMULA USED: Time = Work / Efficiency CALCULATION: Saral, Ravi and Karan alone takes 10, 12 and 15 days Efficiency of Saral =
1
10
units Efficiency of Ravi =
1
12
units Efficiency of Karan =
1
15
units Suppose the number of days taken to complete the task is ‘a’. Saral worked for 2 days, Ravi worked for (a - 3) days and Karan worked for a days. Now,
2
10
+
(a−3)
12
+
a
15
=1 ⇒
1
5
+
a
12
−
1
4
+
a
15
=1 ⇒3
a
20
=1+
1
20
⇒3
a
20
=
21
20
⇒ a = 7 ∴ Number of days taken to complete the task = 7 days