Answer:
40 hours
Step-by-step explanation:
Lets take "x" as the # of hours it takes the lava to reach the sea
since there is a lava flow of 0.5 kilos per hour
we will multiply "x" with 0.5
0.5x
now the tube system is 20 kilometers long
this means that at a rate of 0.5 kilos per hour the lava will reach the sea once it is 20 kilometers deep
so
0.5x=20
divide "0.5" by both sides to eliminate the coefficient
x=40
it will take 40 hours for the lava to reach the sea
Hope this helps!