CONCEPT:
 
-Sequences
-In an A.P, Last term (L)=(n/2)*[2a+(n-1)d] where
a=first term;
d=common difference between the terms;
 n=number of terms


SOLUTION:
Value after 24 years=400+800+1200+.....14400
L=(n/2)[(2a+(n-1)d]
=>144000 = 24/2 [2a + (24-1)*400]
=>a=1400
Hence the value of the certificates in the 13th year = 144+ (13-1)*400
 = $ 6200 (C)