<PRE><B>Question 289131</B>
Let x = L of 90% gas
Then (1200 - x) would be the # of L of 75% gas.
We will end up with 1020L (85% * 1200 L) of overall 100% gas.

.9x + .75(1200 - x) = 1020
.9x + 900 - .75x = 1020   
.15x = 120
{{{highlight(x = 800)}}}

Prove:
800 L of 90% gas (720 L pure)
400 L of 75% gas (300 L pure).
</PRE>