Your first equation is: 4378=2x
x=doughnuts c=chocolate p=plain
Then you solve for x: x=2189
Then make a new equation.
2189=c+p
You know c=978 so you replace it for c.
2189=978+p
Then subtract 978 from both sides
p=1211
Then check:
4378=2(978)+2(1211)
4378=1956+2422
4378=4378
Jo sold 1,211 plain doughnuts.