White and black balls

There are seven white and three black balls in an opaque pocket. The balls are the same size.
a) Randomly pull out one ball. What is the probability that it will be white?

We pull out one ball, see its color, and return it to the pocket. Then
we pull out the ball again. What is the probability that:
b) both balls will be white
c) both balls will be the same color
d) will the first ball be white and the second black?

Correct answer:

a =  0.7
b =  0.49
c =  0.58
d =  0.21

Step-by-step explanation:

$a=7/(7+3)=0.7$

$b=a\cdot 7/(7+3)=0.7\cdot 7/(7+3)=0.49$

$c=a^2+(1-a{)}^2=0.7^2+(1-0.7{)}^2=0.58$

$d=a\cdot (1-a)=0.7\cdot (1-0.7)=0.21$

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