a) $$\frac{1}{4}$$
b) $$\frac{1}{{10}}$$
c) $$\frac{7}{{15}}$$
d) $$\frac{8}{{15}}$$
Answer: d
Explanation:
$$\eqalign{
& {\text{A’s}}\,{\text{1}}\,{\text{day’s}}\,{\text{work}} = \frac{1}{{15}} \cr
& {\text{B’s}}\,{\text{1}}\,{\text{day’s}}\,{\text{work}} = \frac{1}{{20}} \cr
& \left( {{\text{A + B}}} \right){\text{‘s}}\,{\text{1day’s}}\,{\text{work}} \cr
& = {\frac{1}{{15}} + \frac{1}{{20}}} = \frac{7}{{60}} \cr
& \left( {{\text{A + B}}} \right){\text{‘s}}\,{\text{4}}\,{\text{day’s}}\,{\text{work}} \cr
& = {\frac{7}{{60}} \times 4} = \frac{7}{{15}} \cr
& {\text{Remaining}}\,{\text{work}}\, = {1 – \frac{7}{{15}}} = \frac{8}{{15}} \cr} $$
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