SOLUTION: If four litres of petrol containing 15% kerosine are added to another 7 litres of petrol containing 10% kerosine, what percentage of the petrol is kerosine?

The four litres are not pure petrol. They contain:

1.  15% of 4 litres or (.15)(4) = 0.6 litres of kerosine
and
2.  4.0 - 0.6 = 3.4 litres of petrol

Similarly, the 7 litres that are added are not pure petrol either.
They contain:

1.  10% of 7 litres or (.10)(7) = 0.7 litres of kerosine
and
2.  7.0 - 0.7 = 6.3 litres of petrol

So the final mixture of 4+7 or 11 litres of liquid contains 0.6+0.7=1.3
litres of kerosine.  So the fraction of kerosine is

1.3 litres
---------- = ·100% = ·100% = % = % 
 11 litres

Edwin