Given Principal = 64000, Amount= 68921 ROI = 5% p.a
By formula of compound interest for half year compounding
Amount = P* ( 1+R/2*100)^n
Amount after 1st half year = 64000* ( 1+5/200)1 = 64000 * 1.025 =65600
Amount after 2nd half year = 64000* ( 1+5/200)2 = 64000 * 1.0252 =67240
Amount after 3rd half year = 64000* ( 1+5/200)3 = 64000 * 1.0253 =68921
Therefore Amount will be Rs 68921 after 3rd half year or 1 and 1/2 year