unfortunately, the answer is not correct, you will need a total of 1,278 wind turbines in the future, so you need to build additionally 828 wind turbines. You seem to have made the right calculations in terms of the decreases in oil and coal reserves, but not with the new share of those energy sources in a decade. You will need to take into account both effects: The decrease from oil and coal, and the increase of overall energy demand.
When in the past 40% came from oil and the reserves decreased by 10%, it is then 36% of the old energy demand. When the demand increases to 120% but the absolute reserves stay the same, you need to calculate 36% / 120% = 30%. The same way, energy from coal will decrease to 34.5%, resulting in 35.5% share of wind energy in a decade. To come up with the total amount of wind turbines in a decade you need to calculate: 35.5% / 15.0% * 450 turbines * 120%, resulting in 1,278 turbines.
In case you want a more detailed discussion on the math problem, please feel free to contact me directly.
I hope this helps,