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.