Answer
$P(all~4~are~marijuana~users)=0.00083521$
Work Step by Step
$17$% $=0.17$
$P(18-to~25-year-olds~is~marijuana~user)=0.17$
$P(all~4~are~marijuana~users)=P(18-to~25-year-olds~1~is~marijuana~user~and~18-to~25-year-olds~2~is~marijuana~user~and~18-to~25-year-olds~3~is~marijuana~user~and~18-to~25-year-olds~4~is~marijuana~user)$
1) not a single event
2) AND
3) independent events
Use the Multiplication Rule for Independent Events (page 282):
$P(all~4~are~marijuana~users)=P(18-to~25-year-olds~1~is~marijuana~user~and~18-to~25-year-olds~2~is~marijuana~user~and~18-to~25-year-olds~3~is~marijuana~user~and~18-to~25-year-olds~4~is~marijuana~user)=P(18-to~25-year-olds~1~is~marijuana~user)\times P(18-to~25-year-olds~2~is~marijuana~user)\times P(18-to~25-year-olds~3~is~marijuana~user)\times P(18-to~25-year-olds~4~is~marijuana~user)=0.17\times0.17\times0.17\times0.17=0.00083521$