Same thing but a little simpler perhaps:

24:1 = 4% oil
40:1 = 2.44% oil

(1 gal)(4% oil) = x gal(2.44% oil)
x = 1.64 gal

Therefore dilute with gasoline to 1.64 gal.

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2002 E320 4-Matic
2008 Subaru Outback
2009 Subaru Forester