The trick is with the simple interest here: say you have a rate of x simple interest, which means that in one year you will have x*S (with S = sum of money invested) interest to collect. Since it is a simple interest, it isn't capitalized, so in 10 years you basically get 10 times x*S or 10xS.
We are told that this 10xS = 600.
Now, in 5 years you will get 5*xS interest. After 5 years, S will be tripled, making the interest you get per one year x*(3S) or 3xS. For the remaining 5 years you will get 5 * 3xS = 15xS.
Now let's see the total interest:
interest for the first 5 years: 5xS.
interest for the last 5 years: 15xS.
Add them up to get 20xs or 2*(10xS). Since we know that 10xs = 600, the solution to the problem will be 1200 or 2*600.
