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Let Davy's present age be D and Christine's be:
We know that:
D-7=6(C-7) \\ D-7=6C-42 \\ D=6C-35
D+1=2(C+1) \\ D+1=2C+2 \\ D=2C+1

So 2C+1 is equal 6C-35
2C+1=6C-35 \\ 36=4C \\ 9=C

Therefore Christine is 9 and Davy is (2*9)+1=19 years old
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