(A) Fuel efficiency or gas mileage #="Distance traveled"/"Gas used"#

Plugging in given numbers we get

Fuel efficiency or gas mileage #=(400" miles")/(15" Gallon")#

Converting miles to km and gallons to litres we get

Fuel efficiency or gas mileage #=(400xx1.60934" km")/(15xx3.78541" litres")#

#=11.34 " km/l"#, rounded to two decimal places.

(B) #1/2# pound of tea leaves give #30# cups of tea.

#=>1# pound of tea leaves give #30xx2=60# cups of tea.

Converting pounds to gram and cups to ml, we get

#1xx453.592# gm of tea leaves give #60xx236.588237# ml of tea.

#=>1# gm of tea leaves give #(60xx236.588237)/453.592# ml of tea.

#:.0.5# gm of tea leaves give #(60xx236.588237)/453.592xx0.5# ml of tea.

#=>0.5# gm of tea leaves give #15.6# ml of tea, rounded to one decimal place.