Mary saved $100 in her piggy bank at first month.Each month,she adds $20 more than the previous month.What is amount of money saved after 6 months?

In the second month, Mary saves $100 + $20 = $<<100+20=120>>120.
In the third month, she saves $120 + $20 = $<<120+20=140>>140.
In the fourth month, she saves $140 + $20 = $<<140+20=160>>160.
In the fifth month, she saves $160 + $20 = $<<160+20=180>>180.
In the sixth month, she saves $180 + $20 = $<<180+20=200>>200.
Therefore, Mary saved a total of $100 + $120 + $140 + $160 + $180 + $200 = $<<100+120+140+160+180+200=900>>900 after 6 months. Answer: \boxed{900}.