(2) Cut out the Shanghai area depicted by tracing paper with scissors, and then cut out several pieces of white paper according to the shape of tracing paper;

③ Weigh the quality of the white paper representing Shanghai area with a balance;

④ Using the map scale, the actual area of Shanghai can be calculated.

(2) If the mass of white paper representing the area of Shanghai is m, the mass of white paper per square centimeter is ρ, and the map scale is n, then the formula for calculating the actual area of Shanghai is s = n2mρ.

According to the above formula, the mass of white paper per square centimeter is ρ = 8g27× 38cm2 = 7.8×10-3g/cm2;

The blank paper area corresponding to the actual area of 20km2 is s1= (20×1010) cm2 (6×105) 2 = 0.56cm2,

The mass of white paper with an area of S 1 is m1= ρ s1= 7.8×10-3g/cm2× 0.56cm2 = 4.36x10-3g;

Because the balance sensitivity m0=0.05, the corresponding error quality of each piece of paper should not exceed m1= 4.36×10-3g; So at least the number of sheets of white paper is needed.

The actual area of 20 square kilometers corresponds to the area of 0.56 square centimeters of white paper, and the mass of 0.56 square centimeters of white paper is 4.36× 10-3 grams, so at least N=m0m 1=5× 10? 2g4.36× 10？ 3g= 1 1.5 (sheets),

A: At least 12 sheets of white paper are required.