Question 30321
Let's see...the traffic lights are on (green, yellow, or red) for a total of 50 seconds (green) + 5 seconds (yellow) + 25 seconds (red) = 80 seconds.
So, the probability of a green light being on is:
{{{p(G) = 50/80}}}
{{{p(G) = 5/8}}}

The probability of a yellow light being on is:
{{{p(Y) = 5/80}}}
{{{p(Y) = 1/16}}}

The probability of a red light being on is:
{{{p(R) = 25/80}}}
{{{p(R) = 5/16}}}

The probability of A or B is:
{{{p(A or B) = p(A) + p(B) - p(A and B)}}} but for mutually exclusive events such as in this situation (Only one of the lights can be on at any time), {{{p(A and B) = 0}}} so we have:
{{{p(A or B) = p(A) + p(B)}}}

For the traffic lights, the probability of green or yellow being on is:
{{{p(G or Y) = p(G) + p(Y)}}}
{{{p(G or Y) = (5/8) + (1/16)}}}
{{{p(G or Y) = 11/16}}}