Percentile

Need to calculate different percentiles based on different grades. how do I put an 'if' loop or anything else that will work for this.

Grade Amount
B2 3
B2 2
B3 5
B3 6
B4 8
B4 9
B5 10
B5 11
B6 15
B2 3
B3 7
B6 14



Need function for the table below
Percentile 10% 25% 50% 60% 75% 90%
B2
B3
B4
B5