How to make Calculations using two entities in currencies

[lletsweletse]

I have books and i have enter them into my database using microsoft access.

Entities are:Vendor,Book Title,Quantity and price

Student entities:Name,Book title,Book price,Quantity issued,Total allowance

When the book issued to the student i want the quantity to be reducted from the system for book entered.Students are given P2000.00 for book allowance,i want the system to be like this,when entering the book price to reduct the amount from P2000.00 and give me total balance left.I am using Microsoft Access
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I have books and i have enter them into my database using microsoft access.

Entities are:Vendor,Book Title,Quantity and price

Student entities:Name,Book title,Book price,Quantity issued,Total allowance

When the book issued to the student i want the quantity to be reducted from the system for book entered.Students are given P2000.00 for book allowance,i want the system to be like this,when entering the book price to reduct the amount from P2000.00 and give me total balance left.I am using Microsoft Access