百分數...急..THX

一個籃球的成本是$50,標價比成本多70%.若售貨員以八折把籃球出售,,問它的售價是多少?
$50 x (1+70%) x 80% = $68

某人購入10枚金幣,每枚$3000,其後將金幣全數沽出..每枚$2700.
a) 求總虧損,
b) 求虧損百分率.
a) ($3000 x 10) - ($2700 x 10) = $3000
b) $3000/($3000 x 10) x 100% = 10%