Question

a jar is 6/8 full of water.if 3liters is poured making it 3/8full.how much liter will be needed to make the jar full?

Answer 1

So if the content changed from 6/8 full to 3/8, the water was poured out not poured in.
So the change is 3/8 of the capacity

let that capacity be x litres
(3/8)x = 3
x = 3(8/3) = 8 litres

so 5 litres will be needed to make the jar full.

btw, "how much liter will be needed " is poor grammar, say
"how many litres will be needed ..."