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Question Number 122615 by john santu last updated on 18/Nov/20
Commented by liberty last updated on 18/Nov/20
$$\:\frac{\mathrm{1}}{{t}}\:=\:\frac{\mathrm{1}}{\mathrm{12}}\:+\:\frac{\mathrm{1}}{\mathrm{15}}\:−\:\frac{\mathrm{1}}{\mathrm{10}}\:=\:\frac{\mathrm{5}+\mathrm{4}−\mathrm{6}}{\mathrm{60}}\: \\ $$$$\:\frac{\mathrm{1}}{{t}}\:=\:\frac{\mathrm{1}}{\mathrm{20}}\:\Rightarrow\:{t}\:=\:\mathrm{20}\:{hours} \\ $$