Name    Amount     Interest
 JB      10000       0.5
 Jefh    50000       1.25
 John    20000       0.25

Name    Amount     Interest    Total Interest
 JB      10000       0.5        50
 Jefh    50000       1.25       625
 John    20000       0.25       50

awk -F, '$8 > 0 {print $1,",",$2,",",$3,",",$2*$3}

Name    Amount     Interest     0
 JB      10000       0.5        50
 Jefh    50000       1.25       625
 John    20000       0.25       50