Given,
In an arithmetic progression sum of first 20 terms is 790
So, let a be the first term and d be common difference,
So, S20=220[2a+19d]=790.....(1)
And sum of the first 10 terms is 145
So, S10=210[2a+9d]=145.....(2)
By solving equation (1)&(2) we get,
a=-8&d=5
Then, S15−S5=215[2×(−8)+14×5]−25[2×(−8)+4×5]
⇒S15−S5=405−10=395