The ratio of petrol to kerosene in vessel 1 is: 4:3 , So if we take x litres from it then amount of petrol=4/(4+3)*x=4x/7 similarly amount of kerosene=3x/7
If we take y litres from vessel 2 then amount of petrol is 3y/8 and amount of kerosene is 5y/8.
Given: In the mixture volume of petrol=volume of kerosene
4x/7+3y/8=3x/7+5y/8
x/7=2y/8
x/y=7/4
So the ratio in which they should be mixed is 7:4