Jo earned $4378 by selling doughnuts for two dollars each. if she sold 978 chocolate doughnuts and all the other doughnuts she sold were plain how many plain doughnuts did she sell?

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.