Xiaogang's mother is 50 years old, more than 12 years old. Let Xiaogang be x years old?

Xiaogang's mother is 50 years old, more than 12 years old. Let Xiaogang be x years old?

2x+12=50
2x=50-12
2x=38
x=19
Xiaogang is 19 years old

Xiao Li plans to read a 420 page book in January, and she read 84 pages in the first seven days. According to this calculation, can Xiao Li finish the book on schedule?

January 31 day 84 / 7 = 12 12 * 31 = 372 372 〈 420, so we can't finish it on time

Xiao Hong and Xiao Li buy the same book. Xiao Hong is short of $1.40 and Xiao Li is short of $1.60. If they buy the book together, the money is just right. What's the price of the book?
Urgent, require detailed description. The more detailed the better

Suppose the price of this book is x yuan
If Xiao Hong is short of 1 yuan and 4 jiao, she has x-1.4 yuan
If Xiao Li is short of 1 yuan and 60 Jiao, Xiao Li has x-1.6 yuan
If you buy the book with their money, it's just the right money
x-1.4+x-1.6=x
x=3
The price of this book is three yuan

Xiao Li can get to school in time by walking at the speed of 5 km / h. once, after a third of the whole journey, she got on the bus at the speed of 20 km / h
Xiao Li can get to the school in time by walking at the speed of 5km / h. once, after a third of the whole journey, she got on the bus at the speed of 20km / h, so she arrived at the school 15 minutes earlier than the original time. How far is her home from the school?

Set her away from home x kilometers
1/3 X÷5＋2/3 X÷20+ 15/60 =X/5
1/15X+1/30X+1/4=X/5
1/10X=1/4
X = 2.5km