The price of a commodity (in rupees per kilogram) is 100+0.1n on the nth day of 2007 (n=1,2,....100), and then remains constant. On the other hand, the price of another commodity ( in rupees per kilogram) is 89+0.15n, on the nth day of 2007 (n= 1,2,...365). On which date in 2007, will the prices of these two commodities be equal? may 21 april 11 april 10 may 20

Since, At the end of 100th day for the first commodity,  
the price is C1=100+0.1*100
                     = 110, 

after 100th day price remains same.
For 2nd commodity, 

At the end of 100th day, 
                    C2=89+0.15*100 
                         = 104
(for every 10 days, C2 get increased by 1.5 (i.e 10*0.15) So,) 

at 110th day, 
                   C2=104+1.5
                        =105.5
at 120th day, 
                   C2=105.5+1.5
                         =107
at 130th day,
                   C2=107+1.5
                          =108.5
at 140th day, 
                    C2=108.5+1.5
                          =110
So, C1 and C2 are equal at 140th day.
140th day in 2007 is May-20