PLEASE HELP!!! Justin is running a race. He has completed 28 km, which happens to be 70% of the total race. How long is the race?
___ km

Answer is 40km
Explanation:
So in this question we can use a proportion
Since
28km is 70% we can write it equal to 70%
28=70

and since 100% is unknown we can write it equal to x
100=x

Then we put it in proportion form

28/x=70/100

Then cross multiply and divide

28(100)/70=x

Which would equal 40