Current asset 600, current lab 1500. How much we can borrow fm bank with out reducing the current ratio bellow 1:5

With your pre-borrowing CR standing at 2 : 5, I assume* that none of the proceeds "x" of the new debt will be deployed into current assets (e.g. inventory, cash in bank). So start with

CR\ =\ \frac{600}{1,500+x} , and solve for x.

*Can you see why that assumption is necessary? If the new debt proceeds were placed into current assets, it would increase the CR from its present level.