Question 571512
let two numbers be x and y

as per 1st condition;
2x = 3y
or x = 3y/2......(1)

as per 2nd condition 
x+y=20......(2)

put the value of 'x' from (1) into (2) and we get
(3y/2) + y = 20
solve the equation and we get,
y = 8

put the value of 'y' into 2 and we get
x=12

hope this will help you