IIT JEE Advanced 2007 Paper 2

For the given reaction
aG + bH → Products
Rate is directly proportional to [G]m[H]n
When concentration of both the reactants is doubled then, rate is given by
[2G]m[2H]n = 2m2n[G]m[H]n
Given that under these conditions, the rate increases eight times, which means either m = 2 then n = 1 or n = 2 and m = 1.
Again it is given that when the concentration of G is doubled, then rate is doubled which means that m = 1 and n = 2.
And the overall order is m + n = 2 +1 = 3.