The number of girls in 1 is twice that of boys. The average height of girls is 160 cm, and that of boys is 154cm. What is the average height of the class?

(160x2+154) ÷ (1+2) =158 (cm)

(2) Cars travel to and from Party A and Party B at a speed of 30 kilometers per hour and return at a speed of 60 kilometers per hour. Find the average speed of round trip.

(1+1) ÷ (1/30+1/60) = 40 (km/h)

Class 3 A scored 90.6 on the average for the first time when counting the math scores in the exam. Later, it was found that the score of X was 79, but it was wrongly calculated as 97. The second recalculation, the average score is 90.2. How many students are there in this class?

(97-79)÷(90.6-90.2)=45 (person)