maths~`

兩枚郵票的所有可能總值是:

$1.4 + 1.4 = 2.8
$1.4 + 2 = 3.4
$1.4 + 5 = 6.4
$2 + 2 = 4
$2 + 5 = 7

而每個可能總值的概率分別是:

P($2.8) = 2/5 x 1/4
P($3.4) = 2/5 x 2/4 x 2
P($6.4) = 2/5 x 1/4 x 2
P($4) = 2/5 x 1/4
P($7) = 2/5 x 1/4 x 2

兩枚郵票的期望總值
= $2.8 x P($2.8) + $3.4 x P($3.4) + $6.4 x P($6.4) + $4 x P($4) + $7 x P($7)
= $4.72
= $4.7 (至一位小數)

(1.4 + 1.4 + 2 + 2 + 5)/5 * 2 = 4.72