Im stuck on this problem. The answer is eight pounds, but how do you get there?

Four chocolate Waddle doughnuts and a jelly doughnut weigh 20 pounds. Nine chocolate Waddle doughnuts and two jelly doughnuts weigh 43 pounds. How much does a jelly doughnut weigh?

I'm stuck on this problem. The answer is eight pounds, but how do you get there?

Four chocolate Waddle doughnuts and a jelly doughnut weigh 20 pounds.
Nine chocolate Waddle doughnuts and two jelly doughnuts weigh 43 pounds.
How much does a jelly doughnut weigh?