Suppose: the goods carried by the donkey are X bags, and the goods carried by the mule are Y bags.

The mule said, if you give me a bag, I'll pay you twice as much.

2X(x- 1)=y+ 1

( 1)y=2x-3

I'll give you a bag, and we just have so many! "

y- 1=x+ 1

(2)y=x+2

(2) Bring in (1)

x+2=2x-3

x=5

This donkey used to carry five bags of goods.