Lets assume the height of the big rectangle is h and width is 5a then
area of big rectangle is 5ah
Now we can convert the shaded area into two squares i.e. one with base as 2a and height as h and another as base a and height as h so
total shaded area is 2a*h/2 + a*h/2 = 3ah/2
or the shaded area is 30% of the 5ah.