Question 177801
Let the amount of gas when Alan’s tank is full is g
Since at the beginning the tank was   {{{1/4}}} full, and when 6 gallons is added, it was   {{{5/8}}} full, then (  .  g) + 6 {{{(g/4)+6}}} =   {{{5/8}}}. 
Therefore,  {{{((g+24)/4)/g=(5/8)/1)}}}.
Therefore,  {{{5g/8=(g+24)/4}}}.
20g  =  8g  + 192
12g  =   192
g  =  16

Therefore, the gas tank holds 16 gallons.