IIT JEE , AIEEE Forum - Mathematics , Algebra - plz solve this problem(with preferably the fastest method i.e minimal equations)

sourab_MCA

A is twice as efficient as B and together they can do the same amount of work as C and D together in same amount of time.if C and D can complete the work in 20 and 30 days respectively,working alone , then in how many days will A complete the work alone.

shinee

c can complete in 20 days and d in 30 days, so , c and d put together in 12 days
so A nd B can also do it in 12 days
suppose B does the work in x days, then A can do it in x/2 days
1/x+2/x=3/x so both can do it in x/3 days
x/3=12
x=36
so, A can do the work in 18 days

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