May i know why do u have to multiply by 10? Thank you.

The total number of parts is the same before and after, multiplying by 10 gives us total of 90 ( just like the total parts in the after ratio), with both before and after now 90 parts, we can use the information given to us to solve the question, you can use either the 24 or the 104 and you can get the answer, then use the other one to check that your answer is indeed correct

I am still confuse, how do u know after ratio is 90? Thank you

U have A is 7/20 of B, so in terms of ratio A:B is 7:20 and also A is 1/9 of C, so A:C is1:9, to get a combined ratio of A B C normally have to find lcm, but in this case u have 1,so just times 7 to the A:C ratio, u get, A:B:C = 7:20:63, add the parts u get 90

Oh i c, now i understand. Thank you very much.