Area of the 120 mm2 triangle: 
Area = 1/2 x height x base 
120mm² = 1/2 x 10 x base 
Find out the base: 
120mm² = 5 x base 
base = 120/5 
=> base = 24mm. 

We know that two triangles that are similar will have same increase in height and base dimensions. 
Since height is doubled, then base is doubled. 

So :

height is 20mm 
base is 48mm 
The area of a similar triangle with a height of 20 mm is :

\(\dfrac{1}{2}\cdot20\cdot48=480\left(mm^2\right)\)

Answer : 480 mm2.