let

j = Jack's current age

d = Diane's current age

Jack is 20 years older than Diane:

j = 20 + d

In 2 years Jack will be three times Diane's age:

j + 2 = 3(d + 2)

by solving the system of equations:

j = 20 + d

j + 2 = 3(d + 2)

we find:

j = 28 years

d = 8 years

click here to see the step by step solution of the system of equations:

Jack's current age is 28.