Surface Area of a sphere =4*pi*r^2
SA of Earth =4 * 3.141592 x 6,371^2 =5.1 * 10^8 km^2
5.1E8 * 29.2%[percentage of land on Earth] =148,938,826 km^2 -SA of Mars(approx.)
148,938,826 /[4*pi] =11,852,175 km^2 - Take the square root of this.
Sqrt(11,852,175) =~ 3,443 km (approx) -the radius of Mars.
3443 / 6371 [This is Earth's radius] =~54% of Earth's radius is Mars's radius.