Thank you. Btw i got it right till a/b = 6/5 then I concluded that a should be bigger because of the ratio. Where did I go wrong here and how to prevent mistakes like these?

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which is the part, specifically, you did not get ? and what mistake you want to prevent?

Carcass: can you please explain the part after 6/5. If a= 1.2B, isn't a bigger? What could be the numbers, if you could list them for a better understanding?

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This is a tricky question. Always this kind of unit and ten place are

It is best always work conceptually on this even though your feeling is abstract.

Picking numbers could lead you to the wrong way almost immediately

However with a good grasp of theory of numbers they are not that difficult

Now

if you take this part

5A = 6B

A is a multiple of 5 and at the same time the whole number is a multiple of 6

For example 54 is composed by 50+4 which is a multiple of five and a multiple of 6

54 has the unit digit 4 < of the tens digit 5

B is the answer

Another approach is to work in the OE and you simply need to know that

A=1.2B

Conceptually this means that AB are not two different numbers but the same number where the unit digit of the number is 20% less than the tens digit or the other way around

1.2 also means that : 1 is 100 and 2 is 20% more if we think in terms of percentage

I hope this definitely helps