Answer:
x=1
You solve for x by cross multyplying
35 = 1/2h (6+1)
35 = 1/2 h + 7
28 = 1/2h
14 = 1h
14 = h
18 / 60 = 0.3
Charlie has completed 30% of his deliveries
Remember property of exponents

add exponents of same base
ok so
50,000 is 5 times 10^4
so
50000 times 10^15=
5 times 10^4 times 10^15=
5 times

=
5 times 10^19
x/.75=x-56/0.4
0.4x=.75x-42
-0.35x=-42
x=120
I set up a proportion in which x=the tank when it is 3/4 full. When the tank is 3/4 full, solving the proportion tells us the x=120 litres. 120/3=40, and 120+40 is 160, showing that a tank completely full would indeed hold 160 litres.
Hope this helps!