Maths problem: how to do, thanks?

問題有些語焉不詳...或許是我理解能力較差?

假設題意是: 經一年(a)或2年(b)一次還清本利和?

如是, 則:
(a)
總利息 = 6500×[(1+4%/4)^4 -1] + 4000×[(1+6%/12)^12-1]
            = 6500×0.04060401 + 4000×0.0616778119
            = 264 + 247 = 511
(b)
總利息 = 6500×[(1+4%/4)^8 -1] + 4000×[(1+6%/12)^24-1]
            = 6500×0.0828567056 + 4000×0.1271597762
            = 539 + 509 = 1o48