Mary saved $100 in her piggy bank at first month. Each month, she adds $20more than the previous month. How much money will be added in the piggy bank at 6th month?

In the second month, Mary will add $100 + $20 = $<<100+20=120>>120.
In the third month, she will add $120 + $20 = $<<120+20=140>>140.
In the fourth month, she will add $140 + $20 = $<<140+20=160>>160.
In the fifth month, she will add $160 + $20 = $<<160+20=180>>180.
Therefore, in the sixth month, she will add $180 + $20 = $<<180+20=200>>200. Answer: \boxed{200}.