My result: 33
a/7=20 => a= 140 (sum of the 7 numbers)
b/13=40 => b=520 (sum of the 13 numbers)
(a+b)/20 = c =660/20 =33 (the sum of the 20 numbers is 660, divided through 20 makes the mean of the 20 numbers)

I found only one solution:
Paul is 18, Patricia is 20 and Daniel is 25 years old.
My way of solution:
a= Paul, b=Patricia, c=Daniel
I: a+2=b
II: 1.25*b=c
III: c-10=1.5*(b-10)
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II+III: 1.25b=1.5*b-(1.5*10)+10 => b=20
II': 1.25*20=c=25
I': a=b-2=20-2=18