Mine

table "a"

cust_id  bill_amt   total
1        100        300
2        120        120
3        50         100
4        75         225 
1        100        300
3        50         100
4        75         225
1        100        300 
4        75         225

here total for 1 is 300( 3 times 100 rs amt given by cust 1)