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The first volume of the sixth grade math book exercises 15 answers to questions 7, 8, 9 and 10.
7.100 ÷ 4×1/2 =12.5 (cm)

8.40cm = 0.4m.

50÷(3. 14×0.4)≈40 (weeks)

9.50× 4+3.14× 50 ÷ 2 = 278.5cm = 2.785m.

10. The perimeter of the shaded part is the sum of half the perimeter of the big circle and the perimeter of the small circle.

2× 3.14× (5÷ 2)+3.14× 5 = 31.4 (cm)