J can complete a work in 21 days, K can complete it in 42 days and L can complete it in 63 days. If J, K and L work together, in how many days can they complete the work?

Let's assume the total work is 126 units.

J can complete a work in 21 days.

Efficiency of J = $$\frac{126}{21}$$ = 6 units/day

K can complete it in 42 days.

Efficiency of K = $$\frac{126}{42}$$ = 3 units/day

L can complete it in 63 days.

Efficiency of L = $$\frac{126}{63}$$ = 2 units/day

If J, Kand L work together, then the time taken by all of them to complete the work = $$\frac{126}{6+3+2}$$

= $$\frac{126}{11}$$

= $$11\frac{5}{11}$$ days