* * * Need

4 ÷ 2/5 = 10 (hour)

Still need

10-4 = 6 (hours)

Method 2

Injection every hour

2/5÷4= 1/ 10

* * * Need

1÷110 =10 (hours)

Still need

10-4 = 6 (hours)

Method 3

Injection every hour

2/5÷4= 1/ 10

You also need an injection.

1-2/5=3/5

Still need

3/5 ÷110 = 6 (hours)

Method 4

You also need an injection.

1-2/5=3/5

Still need

3/5 ÷ 2/5× 4 = 6 (hours)

Method 5

* * * Injection

600× 2/5 = 240 (cubic meter)

Injection every hour

240 ÷ 4 = 60 (cubic meters)

* * * Need

600 ÷ 60 = 10 (hour)

Still need

10-4 = 6 (hours)

Method 6

* * * Injection

600× 2/5 = 240 (cubic meter)

Injection every hour

240 ÷ 4 = 60 (cubic meters)

Also inject

600-240 = 360 (m3)

Still need

360 ÷ 60 = 6 (hours)