How will I get 100 fruits in Rs500
1watermelon= Rs50
1 apple= 10Rs
1 banana= 1Rs
w+a+b = 100
50w+10a+b = 500
The equations don't work out too well, so let's play around with some values that add up to Rs500, and see whether we can wind up with 100 fruits:
(b,a,w)
90,1,8 = 500
91,1,8 = 501
50,40,1 = 500
50,30,3 = 500
Since the first line shows 99 fruits adding up to 500, I doubt you can do better than that. Replacing one fruit with another changes the total by more than Rs1.