[tex]\qquad\qquad\huge\underline{{\sf Answer}}♨[/tex]
If temperature remains constant, then we can apply the formula :
[tex]\qquad \tt \dashrightarrow \:p_1v_1 = p_2v_2[/tex]
[tex]\qquad \tt \dashrightarrow \:4 \times v_1 = 6 \times 18[/tex]
[tex]\qquad \tt \dashrightarrow \: v_1 = 108 \div 4[/tex]
[tex]\qquad \tt \dashrightarrow \:v_1 = 27[/tex]
So, the required volume is 27 litres