A certain sum of money lent out at simple interest amounts to Rs.1,550 in three years and Rs.1,750 in five years. Find the principle and the rate of interest.

In 5 yrs=1750        
increase in two years=1750-1550=200     

So, increase in 1 year=200 /2=100        
Thus,increase in 3 years=100*3=300

principal=1550-300=1250              
rate = interest*100 / principal* time