Awater tank has a radius of 80cm and a length of 200cm. it is half filled with a water. 120 litres per a day is used to water a garden. for how many days can the garden be watered from the tank ?

The cylinder has volume •πR'2h•
So,transfer radius and length in metre values.
So:
R = 0.8m
L = 2m
V = πR'2h = 12.8π ~ 40.21(m'3)
The half of V = 40.21/2 ~ 20.1(m'3)
1m'3 has 1000 litres.
So, 20.1m'3 have 20.1*1000 = 20.100 litres.
20.100/120 = 167.5 days.
Answer: 167.5 days.