Time & Work

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Answer:

You are given:

A + B + C = 5

B + C + D = 10

C + D + A = 15

D + A + B = 30

Add all : (A + B + C + D) appears 3 times in total.

So:

3(A+B+C+D)=60

$$=60$$

Now divide by 3: A+B+C+D=20​

Answer: 20 days