Mark (✓) against the correct answer:
Question:

Mark (✓) against the correct answer:

A can finish a piece of work in 12 hours while B can finish it in 15 hours. How long will both take to finish it, working together?

(a) 9 hours

(b) $6 \frac{2}{3}$ hours

(c) $6 \frac{3}{4}$ hours

(d) $8 \frac{1}{3}$ hours

 

Solution:

(b) $6 \frac{2}{3}$ hours

A’s 1 hour work $=\frac{1}{12}$

B’s 1 hour work $=\frac{1}{15}$

$(\mathrm{~A}+\mathrm{B})$ ‘s 1 hour work $=\frac{1}{12}+\frac{1}{15}=\frac{9}{60}=\frac{3}{20}$

Time taken by $\mathrm{A}$ and $\mathrm{B}$ to complete the work together $=\frac{20}{3}=6 \frac{2}{3}$ hours

 

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