The formula for the first n terms of a geometric series is: Sum = a · (1 - rn) / (1 - r)
where a is the first term and r is the common ratio.
We have: 10 = a · (1 - r10) / (1 - r) and 70 = a · (1 - r30) / (1 - r)
combining: [ a · (1 - r30) / (1 - r) ] / [ a · (1 - r10) / (1 - r) ] = 70 / 10
reducing: (1 - r30) / (1 - r10) = 7
rewriting: ( r30 - 1 ) / ( r10 - 1 ) = 7
If we divide ( r30 - 1 ) by ( r10 - 1 ) we get r20 + r10 + 1
so: r20 + r10 + 1 = 7
and: r20 + r10 - 6 = 0
Let x = r10 ---> x2 = r20
so: r20 + r10 - 6 = 0 ---> x2 + x - 6 = 0 ---> (x + 3)(x - 2) = 0
x can't be negative, so x = 2 ---> r10 = 2 ---> r = 21/10 ---> r = 1.071773463...
Since a · (1 - r10) / (1 - r) = 10 ---> a · (1 - 1.07177346310) / (1 - 1.071773463) = 10
Solving: a = 0.7177346254 [ or a = -10(1 - 21/10) ]
To find the sum of the first 40 numers: Sum = a · (1 - r40) / (1 - r) = 150