**Solution-**

The C.P. of $15 \mathrm{~kg}$ of milk $=$ Rs. $7 \times 15=$ Rs. 105

To gain Rs. 64 ,

Total S.P. = Rs. $105+$ Rs. 64

= Rs. 169

Now there is $(15-2)=13 \mathrm{~kg}$ of milk

$\therefore$ The S.P. of $13 \mathrm{~kg}$ of milk

= Rs. 169

$\therefore$ Required S.P. = Rs. $169 \div 13$

= Rs. 13 ; Ans.