For $\mathrm{A}+\mathrm{B}\to \mathrm{C}+\mathrm{D}$,when [A] alone is doubled, rate gets doubled. But, when [B] alone is increased by 9 times, rate gets tripled. Then, order of reaction is

When A= doubled $\Rightarrow$rate=doubled (since, 2¹=2)

i.e. Order w.r.t A=1

When B= increases by 9(3²) times $\Rightarrow$ rate=tripled 

i.e. order w.r.t B=1/2  {since, (3²)^1/2=3}

$\therefore$Order of the reaction= 1+1/2=3/2.