Compound Interest (CI) is the interest earned on the principal amount and also on the accumulated interest of the previous periods.

Formula for Compound Interest (CI) = P (1 + r/n) ^ nt
Where, P = Principal Amount
r = Rate of Interest
n = Number of times the interest is compounded
t = Time in years

Given,
P = Rs 6950
r1 = 6% p.a. for the first two years
r2 = 9% p.a. for the third year
n = 2 (half-yearly)
t = 3 years

Calculation:
Compound Interest (CI) for first two years
= P (1 + r1/n) ^ nt
= Rs 6950 x (1+ 6/2) ^ (2 x 3)
= Rs 6950 x (1 + 3) ^ 6
= Rs 6950 x 729
= Rs 50,530

Compound Interest (CI) for third year
= P (1 + r2/n) ^ nt
= Rs 6950 x (1 + 9/2) ^ (2 x 1)
= Rs 6950 x (1 + 4.5) ^ 2
= Rs 6950 x 20.25
= Rs 140,612.50

Total Compound Interest (CI)
= CI for first two years + CI for third year
= Rs 50,530 + Rs 140,612.50
= Rs 191,142.50

Compound Interest (CI) for 3 years
= Total CI - Principal Amount
= Rs 191,142.50 - Rs 6950
= Rs 184,192.50

Compound Interest (CI) for 3 years
= CI for 3 years - CI for first two years
= Rs 184,192.50 - Rs 50,530
= Rs 133,662.50

Therefore, the compound interest on Rs 6950 for 3 years if interest is payable half-yearly, at the rate of 6% p.a. for the first two years and at the rate of 9% p.a. for the third year is Rs 1590.

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