45.8 is 2.65 less than 5.1 times of a number

45.8 is 2.65 less than 5.1 times of a number

Let this number be X
5.1X-45.8=2.65
5.1X=48.45
X=48.45/5.1
X=9.5
This number is 9.5

8 / 5 of 12 is 2 / 2 less than 5 times of a number. Find the number (by solving the equation)

Let this number be X
therefore
12×8/5=5x-1/2
96/5=5x-1/2
5x=96/5+1/2
5x=197/10
x=197/50

5 / 8 of a number is 15 more than 2 / 2 of 12. Find the number (the solution of the equation)

Set to X
Then:
x*5/8-12*(5/2)=15
5x/8-30=15
5x/8=45
x/8=9
x=72

5 times of a number is 12 less than 5 / 8 of 120. To find this number, use the equation

Let this number be X
therefore
5x = 120 × 5 / 8-12
5x=75-12
5x=63
x=63÷5
x=12.6

Calculation: 1999.9 + 199.9 + 19.9 + 1.9=______ ．

1999.9 + 199.9 + 19.9 + 1.9 = (2000-0.1) + (200-0.1) + (20-0.1) + (2-0.1) = 2222-0.1 × 4 = 2222-0.4 = 2221.6, so the answer is: 2221.6

Observe a string of numbers: 3, 5, 7, 9 The nth number can be expressed as ()
A. 2（n-1）B. 2n-1C. 2（n+1）D. 2n+1

3，5，7，9… It is a continuous odd number starting from 3, so it is represented by 2n + 1, so D

9 and 1 / 3 (49 / 12-63 / 20 + 77 / 30-91 / 42 + 105 / 56)

9 and 1 / 3 - (49 / 12-63 / 20 + 77 / 30-91 / 42 + 105 / 56)
=9+1/3-（7/3+7/4-7/4-7/5+7/5+7/6-7/6-7/7+7/7+7/8）
=9+1/3-（7/3+7/8）
=9+（1/3-7/3）-7/8
=9-2-7/8
=7-7/8
=6 and 1 / 8

The bottom of a triangle is 8.2 cm. If the bottom is increased by 4 cm, the area will be increased by 10 square cm
Area

Height h = 10x2 / 4 = 5
S＝5x8．2／2＝20．5

2x △ 5-24 = x △ 4 to solve the equation

8x-480=5x
x=160

A semicircle with an arc length of 6.28 cm. The circumference of the semicircle is______ Cm, the area is______ Square centimeter

6.28 △ 3.14 = 2 (CM), 3.14 × 2 + 2 × 2, = 6.28 + 4, = 10.28 (CM), 3.14 × 22 △ 2, = 3.14 × 2, = 6.28 (square cm). Answer: the circumference of this semicircle is 10.28 cm, and the area is 6.28 square cm