Her card has a $150.00 balance and she makes a $50.00 payment so the new balance is 150-50 = $<<150-50=100.00>>100.00
She didn't pay it off so she is charged 20% interest on her $100.00 balance so the interest is .20*100 = $20.00
Her balance was $100.00 and she was charged $20.00 in interest so her new balance is 100+20 = $<<100+20=120.00>>120.00
#### 120