After earning interest, the balance of an account is $420. the new balance is 7/6 of the original balance. how much interest did it earn

420=7/6x

The 6, who was under / now goes across the equal sign with * value.
420*6 =7x
2520 = 7x
Now the 7, who is under * value, goes across the equal sign with / value, so
2520/7 = x
x= 360.

420-360 = 60
It earned $60 of interest.