The probability to obtain a $3$ is $p$, and the probability that there is at least one $3$ in four tosses is $0.9375$. Find $p$. I have this simple problem but, I have some problems with it:
A die is biased and the probability to obtain a $3$ is $p$. The probability that there is at least one $3$ in four tosses is $0.9375$. Find $p$.
I have tried something but without success. For example, the probability to obtain at least one 3 in 4 tosses, is the same to say the probability to not obtain zero 3 in 4 tosses.  
$1-(1-p)^4 = 0.9375$ 
or also I have considered, with n=4:
$1-\sum_{k=1}^4 \binom{4}{k}(p)^k(1-p)^{4-k}$
can you help me? Thanks!

$$\begin{array}{lcl} 1−(1−p)^4 & = & 0.9375 \\ (1−p)^4 & = & 1-0.9375 \\ (1−p)^4 & = & .0625 \\  1−p & = & .0625^\frac{1}{4} \\ p & = & 1-.0625^\frac{1}{4} \\ p &  = & 0.5\end{array}$$
