This problem is a combination of numbers.
1 * 2 = 22 * 2 = 4 124.
3 * 2 = 6 6 * 2 = 12 constitutes 36 12.
5 * 2 = 10 10 * 2 = 20 makes up 5 1020.
See that the first number has increased by 2.
So the number of the next head is 7.
28 groups in 7 * 2 =1414 * 2 = 71428.
2.87，57，36， 19，(D)， 1 a . 17； b . 18； c . 16； D. 10
87 8*7=56 56+ 1=57
57 5*7=35 35+ 1=36
36 3*6= 18 18+ 1= 19
19 1*9=9 9+ 1= 10
10 1*0=0 0+ 1= 1
This is to separate two numbers, multiply them and add one.