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.