the limiting sum of a geometric series is used when a series converges...
so
the formula is
S∞=a1−r
a = 1st term and r is the common ratio. the value of r will be |r|<1
this is what makes it converge
In your question you will need to find r, the common ratio to allow you to find the 4 terms
so using your information
10=21−r
1−r=15
which will result in
r=45
so to find the 2nd, 3rd and 4th terms use the formula
Tn=arn−1
you know the 1st term
T2=2×45
T3=T2×45
T4=T3×45
hope this helps