Symbiosis National Aptitude Test SNAP 2016 Paper

Let B be closed after x min. then,
Part filled by (A + B) in x min. +part filled by A in (18 – x) min. = 1
Therefore x × ((1/24) + (1/32)) + (18 – x) × (1/24) = 1
(7x/96) + ((18 – x)/24) = 1.
7x + 4 × (18 – x)=96.
⇒ 3x = 24
∴ x = 8
Hence, B must be closed after 8 min.