A and B earn Rs. 320 in 4 days, B and C earn Rs. 450 in 10 days. A, B and C earn Rs. 600 in 5 days. Find the daily earning of B.

1 day earnings of $A\&B=\frac{320}{4}=80$

1 day earning of  A&C$=\frac{450}{10}=45$

1 day earnings of A, B&C $=\frac{600}{5}=120$

A + B = 80..........(i)

B + C = 45..........(ii)

A + B + C = 120............ (iii)

(i)  + (ii) – (iii)

(A + B) + B + C – (A + B + C) = 45 + 80 – 120

A + 2B + C – A – B – C = 125 – 120

  B = 5