The total amount of interest will be found using compound interest formula:
A=p(1-r/100)^n
A=future amount
p=principle
r=rate
n=time
From the information given:
p=$6500, r=6%, n=25 years
thus
A=6500(1+6/100)^25
A=6500(1.06)^25
A=$27,897.16
Answer: 27897.12-6500=$$21397.2