sheep herd

As follows:

There are sheep and goats in a sheep herd. It is known that the sheep constitute 45% of the herd, and their total weight constitutes 55% of the total weight of the herd. The average weight of a goat is 81 kg. What is the average weight of a sheep?

Let's suppose that we have 100 animals and that 45 are sheep and 55 are goats

Then....the total weight of all the goats =  55 *81  = 4455kg

And the sheep  constitute  55/45 of this weight =  (55/45)  (4455)  = (11/9) (4455) = 5445 kg

So...the avg weight of a sheep  =   5445 / 45  =  121 kg