Ans. 15 % p.a.
Explanation: Let, the actual rate of interest be r% p.a. and principal be Rs. 100.
So, in 1st case, after 2 yrs, the interest = Rs. 2r .
In 2nd case, rate of interest = (r + 6)% p.a.
By the problem, 2(r + 6) = (140/100)*2r  .       => 2r + 12 = 2.8r   .        =>  r = 15 .
Hence, the reqd. rate of interest = 15% p.a.