Let Rs. x part be lent at 8% simple interest ⇒ Rs. (20000 - x) Part is lent at 8/3 % simple interest. Now, as per given total simple interest after an year = Rs. 800 ⇒
x×8×1
100
+
(20000−x)×
8
3
×1
100
=800 ⇒ (24x + 160000 - 8x)/300 = 800 ⇒ 16x + 160000 = 240000 ⇒ 16x = 80000 ⇒ x = 80000/16 = Rs. 5000 ∴ the sum lent at 8/3 % = (20000 - 5000) = Rs. 15000