by a reduction of 1kg in the price of sugar, mohan can buy one kg sugar more for Rs56 find the original price of sugar

If the price is now p, then he can buy 56/p kg. So, at the new price,

(p-1)(56/p + 1) = 56
p = 8

check:
mohan can buy 7kg at Rs8/kg
or, 8kg at Rs7/kg