Question

A is twice as fast as B and takes 20 days to do a job. In how many days ca A and B do it together?

Answer 1

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We have to find in how many days they will together will complete the whole work ???

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$\huge\blue{</strong><strong>Given</strong><strong>}$ :-- A is Twice as fast as B .

That means Efficiency of A : B = 2 : 1

and Their no. of Days Ratio will be = 1 : 2

(As days is inversely proportional to Efficiency)

so, when A takes 20 days , B will will take 40 days.

Together They will complete whole work in =====

$( \frac{1}{20} + \frac{1}{40} ) = \frac{3}{40} = \frac{40}{3} = 13( \frac{1}{3})days$

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Answer 2

A_20

B_40

lcm of 20,40= 40

so

A=40/20=2units work

B=40/40=1unit work

total=units work /day

so take 40/3days=13day and 1/3day