Pls tell gst

Solution: 
Purchased goods Cash Book Transaction :
= 40,000 - 6,000(trade discount @ 15%)
= 34,000 (Purchased value) Credit side of Bank column 
IGST @12% : 34,000*12% = 4,080 Credit side of bank column

Sales Cash Book transaction :
= 30,000 - 3,000(trade discount @10%)
= 27,000 (Sale value) Debit side of bank column 
IGST @ 12% : 27,000*12% = 3,240 debit side of bank column