[tex]\frac { 4 }{ 9 } =2M\\ \\ \frac { 1 }{ 2 } \cdot \frac { 4 }{ 9 } =\frac { 1 }{ 2 } \cdot 2M\\ \\ \frac { 4 }{ 18 } =M\\ \\ \therefore \quad M=\frac { 2 }{ 9 } [/tex]
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Every minute 2/9 of the tank is filled up. That means that every half minute 1/9 of the tank is filled up.
2/9 + 2/9 + 2/9 + 2/9 + 1/9 = 9/9
Therefore:
M + M + M + M + 1/2M = 9/9
(4+1/2)M=9/9
So it would take 4 and a half minutes to fill the whole gas tank.
