This is a "trial and error" type question. Algebra alone won't help you.
First see what you could do buying 19 cow brains ($95) with $5 left. That could get you ten sheep brains ($1) and two pig brains ($4). The total would be only 31 total. Not enough. So try getting fewer cow brains. With 18 cow brains ($90) you can get 80 sheep brains ($8) and 1 pig brain ($2). Not quite 100 total. The number of sheep brains will have to be divisible by ten to come out with an even number of dollars.
With those rules in mind, see what other combinations might work
On math sheet says.
I have to spend 100.00 and have a hundred brains. There are sheep brains wich are .10cents and then pig 2.00 and then cow 5.00. I have to at least by one of each brain.
I need major help