x*0.4%*6=7.8
0.024x=7.8
x=325
A: The library books are worth 3.25 million yuan.
2. Solution: Suppose the original X people, including 4x/7 boys and 3x/7 girls.
4x/7=2(3x/7- 12)
4x/7=6x/7-24
4x=6x- 168
6x-4x= 168
2x= 168
x=84
4*84/7=48 (person)
A: There are 48 boys.
3. Solution: Suppose the original price of each set is X yuan.
x *( 1- 10%)*( 1- 10%)= b
x*0.9*0.9=b
0.8 1x=b
x= 100b/8 1
A: The original price of each set is (100b/8 1) yuan.
4. solution: suppose you want to intercept X cm.
(4/2)^2*3. 14*x=3*(2/2)^2*3. 14* 16
12.56x= 150.72
x= 12
Answer: round steel 12cm needs to be intercepted.
5. Solution: Assume that the sum of decimal points is actually X yuan, and the decimal point is 0. 1x yuan.
x-0. 1x=273.6
0.9x=273.6
x=304
A: The total number of wrong decimal points is actually 304 yuan.
6. Solution: Suppose that the puppy ran X kilometers at this time.
x/ 16=4/( 12-4)
x/ 16= 1/2
x= 1/2* 16
x=8
A: The puppy ran 8 kilometers at this time.