When the temperature of a reaction is increased by 20^oC, the rate of reaction increases by

4 times

It has been found that for a chemical reaction with rise in temperature by 10°, the rate constant is nearly doubled.

Hence rate of reaction increases.

Let at T^oC, Rate of reaction, R = k [A]

At T + 10^oC, Rate of reaction, R' = 2k[A]

At T + 12^oC, Rate of reaction, R'' = 2(2k[A]) = 4k[A] 

When the temperature of a reaction is increased by 20^oC, the rate of reaction increases by 4 times.