You said .... is less than or equal to 3, but you only summed less than
And you don't even say what your i is.
Let X have a binomial distribution with the number of trails n = 10 and with p either 0.25 or 0.5. The simple null hypothesis p = 0.5 is rejected and the alternate hypothesis p = 0.25 is accepted if the observed value of X1, a random sample of size 1, is less than or equal to 3. Find the significance level and the power of the test.
I believe that the power function is P(X<3) = P(Bin(10,0.25) < 3)and the significane leve would be
Is this correct?