Explanation:

=> tin of oil capacity = 4/5
=> no. of bottles taken from it = 6
=> no. of bottles poured into it = 4
=> no. of bottles which were full = 3/4
So bottles left after pouring = total bottles - bottles which were poured
= 6 - 4 = 2
So 4/5 is total no. of bottles and bottles left is 3/4 and bottles left were 2
=> considering the following LHS numbers as x evaluate or simplify them
=> 4/5x - 3/4 x = 2
=> 16/20 x - 15 / 20 x = 2 ( first find LCM of denominators which get 20 So convert both the fractions by changing their numerator and denominator and making them like fractions
=> 1/20 x = 2
=> 1x/ 20 *20 = 2 x 20
=> 1x = 40 ( cancel 20 in LHS and multiply on RHS )
=> x = 40 ( If you divide 1 on both RHS and LHS then the solution will be same )
So total bottles are 40
4/5 of 40 we will obtain our total = 32
3/4 of 40 we will obtain our bottles left = 30