Arithmetic progression question

[VAISHU]

Dear friends,

Please help me answers the question for me due to I found it was confusing and difficult for me to answers it.

Regards,

Navi

[Guest]

a) 20000 x 0.06 x t = 10000
1200t = 10000
t = 8.33 years if simple interest

b) a + 8d = a + a + d + a + 2d + a + 3d
a + 8d = 4a + 6d
2d = 3a
a = 2/3 d