(2) As can be seen from the figure, the scores are [89,99], [99, 109], [109,19], [1 19] respectively.

The frequencies are 0.05, 0. 15, 0. 15, 0.30, 0.25, 0. 10 respectively.

∴ The average grade of these 60 students is: 94× 0.05+104× 0.15+14× 0.15+124× 0.30+6544.

(3) As can be seen from the figure, the frequencies of the scores in [1 19, 129], [129] and [149] are 0.30 respectively.

Therefore, if the students whose grades are between [1 19, 149] are taken as a whole, 26 students are selected by stratified sampling to analyze the test papers.

Then the extraction ratio in [1 19, 129], [129, 139], [139, 149] is 0.39.

So the number of people with grades between [129, 139] is 26× 5 13 = 10.