You have invested 1200 dollars in your mutual fund and now gained 1333 dollars. First let's solve how much money added to your investment.
=> 1333 - 1200 = 133 dollars was added to your account
=> 133 / 1200 = 0.11
Next, lets convert this to decimal value.
=> 0.11 * 100% = 11% was added to you investment.