The sum of the areas of the 10 squares of the lengths of who sides are 20 cm, 21 cm, ........... 29 cm respectively is

sides of the 10 squares are 20,21,22,............,29 respectively

area of the square = $$ side^2 $$

area of the 10 squares are $$ 20^2,21^2,22^2,..........,29^2 $$ = sum of squares of first 29 natural numbers - sum of squares of first 19 natural numbers

 sum of squares of first n natural numbers = $$ \frac{ n(n+1)(2n+1)}{6} $$

sum of squares of first 29 natural numbers = $$ \frac{ 29(29+1)(2 \times 29 + 1) }{6} = \frac{ 29 \times 30 \times 59}{6} = 8555 $$

sum of squares of first 19 natural numbers = $$ \frac{ 19(19+1)(2 \times 19+1)}{6} = \frac{19 \times 20 \times 39}{6} = 2470 $$

area of the 10 squares are $$ 20^2,21^2,22^2,..........,29^2 $$ = 8555 - 2470 = 6085