Shortcut Trick(A + B) can do 80% of work in 20 days
(A + B) can do 100% of work in 20 × 5/4 = 25 days
Let the total work by 450 units
Efficiency of A = 450 ÷ 45 = 10 unit/day
Efficiency of (B + C) = 450 ÷ 30 = 15 unit/day
Efficiency of (A + B) = 450 ÷ 25 = 18 unit/day
So, Efficiency of B = 8 unit/day
Efficiency of C = 7 unit/day
Since, A + B completed 80% work
So, Remaining work = 20%
Now, 70% of remaining total work = 450 × (1/5) × (7/10) = 9 × 7
∴ Required time taken by C = (9 × 7)/7 = 9 days.
Alternate Method
Given:
A can do a piece of work = 45 days
B and C together can do a piece of work = 30 days
Formula used:
W = E × T Where, W = Work, E = Efficiency and T = Time
Calculation:
Let the C alone can complete the work in C days and B alone can complete the work in B days
The efficiency of A = \({1\over 45}\)
A and B together can complete the 100% work together = 25 days
⇒ \({1\over 45}\ +\ {1\over B}\ =\ {1\over 25}\)
⇒ \({1\over B}\ =\ {1\over 25}\ -\ {1\over 45}\)
⇒ \({1\over B}\ =\ {9\ -\ 5\over 225}\)
B can complete the work = \({225\over 4}\) days
⇒ \({4\over 225}\ +\ {1\over C}\ =\ {1\over 30}\)
⇒ \({1\over C}\ =\ {1\over30}\ -\ {4\over 225}\)
⇒ \({1\over C}\ =\ {15\ -\ 8\over 450}\)
C can complete the work = \({450\over 7}\) days
The 20 days work of for A and B = \({20\over 45}\ +\ {80\over 225}\ =\ {100\ +\ 80\over 225}\ =\ {180\over 225}\)
The remaining work = \({45\over 225}\ =\ {1\over 5}\)
The 70% of the work = \({7\over 10}\ × {1\over 5}\ =\ {7\over 50}\)
C will complete the 70% of the remaining work = \({7\over 50} × {450\over 7}\ =\ 9\ days\)
∴ The required result will be 9 days.