Young McDonald's Animals
When Old McDonald was young and had just started his farm, animals were cheaper then. Each cow costed $10, every pig $3, and every chicken $0.50 [they ate a lot of chicken].
When Young McDonald had started his farm, he spent exactly $100 and had bought 100 animals.
How many chickens did he buy when he first started his farm?
Assume he bought a non-zero number of cows, pigs, and chickens.