X×5/7= 15/28。

Solution: x=3/4

So the correct answer is: 3/4÷5/7=2 1/20.

2. If there are X red balls, there are x+30 yellow balls.

3(x+ 1)=x+30+ 1

Solution: x= 14

There are 14+ 1= 15 red balls and 14+30+ 1=45 yellow balls.

3. Let the speeds of Party A and Party B be 3x and 4x respectively.

The speed ratio between B and C is 2:3, so the speed of C is 6x.

So the speed ratio of A, B and C is 3x: 4x: 6x = 3: 4: 6.

Let A be 5x, then B and C are 6x and 3x respectively.

The average of A, B and C is 70, so the sum of the three numbers is 2 10.

Namely: 5x+6x+3x=2 10.

Solution: x= 15

So A is 75, B is 90 and C is 45.