Formula needed for a Taxi booking log

Hello,

I need two formulae in the attached excel sheet (Taxi booking) under following conditions.

1) I need a Formula for the Cell L4. The taxi fare is Rs 100 for first 4Km and then Rs 12 per additional KM. The present formula being used is not perfect.

2) Also need a formula for cell P4. In case cash has been received then P4 should not calculate the service tax (Cell M4).

Kindly help

Best Regards

Rajiv