∴a=220=0. 1,b=3，

The frequency with a score of [1 10, 150] is 520=0.25, so the frequency with a score of [90, 1 10] is 1-0.

Therefore, the number of students with scores in the range of [90, 1 10] is 20×0.4=8.

According to the stem-leaf diagram in the question, the number of students with scores in the range of [100,10] is 4, so the number of students with scores in the range of [90, 100] is 4.

As can be seen from the frequency distribution table in the question, the frequency with a score in the [70,90] range is 0.25, so the number of students with a score in the [70,90] range is 20×0.25=5, and the number of students who pass the math exam is 13.

Therefore, it is estimated that the passing rate of all students' math scores in this exam is 1320× 100%=65%.

(2) According to the stem leaf diagram, there are five students whose scores are greater than or equal to 1 10. The scores of these five students are 1 16, 1 18, 128, respectively.

There are C25= 10 ways to choose two people from them.

The average score of the two candidates is not less than 130 (142,136); ( 142, 128); ( 142, 1 18); (136, 128), 4 selection method,

The probability that their average score is not less than 130 is 4 10 = 25.