STEP 1:
find number of liters in 30 deciliters
1 deciliter= 0.1 liter
multiply total number of deciliters (30) by liters in one deciliter (0.1).
=30 * 0.1 liter
=3 liters
STEP 2:
subtract 1 liter from step 1 answer because he already has one liter
= 3 liters - 1 liter
= 2 liters still needed
ANSWER: He needs to buy 2 liters
Hope this helps! :)