30(x- 15)=20(x+5)

30x-450=20x+ 100

30x-20x= 100+450

10x=550

x=55

Five minutes in advance, the speed of the motorcycle

(55- 15)×30÷(55-5)

=40×30÷50

= 1200÷50

=24

A: If you plan to arrive five minutes early, the speed of the motorcycle should be 24 kilometers per hour.

7. Let the original 40% brine be x grams and the original 10% brine be y kilograms.

40%x+ 10%y=30%(x+y)①

40% x+ 10% y+300×20% = 25%(x+y+300)②

① Simplified calculation is 0.4x+0.1y = 0.3x+0.3y..

0.4x-0.3x=0.3y-0. 1y

0. 1x=0.2y

x=2y③

② The simplified calculation is 0.4x+0.1y+60 = 0.25x+0.25y+75.

0.4x+0. 1y-0.25 x-0.25y = 75-60

0. 15x-0. 15y = 15

x-y= 100④

③ Substitute into ④ to get 2y-y= 100.

y= 100

x=2× 100

x=200

A: The original 40% salt solution is 200 grams.

8. Suppose that the working hours for making 1 shirts are x, the working hours for making 1 pants are 2x and the working hours for making 1 tops are 3x.

2x+3×2x+4×3x= 10

2x+6x+ 12x= 10

20x= 10

x=0.5

2x= 1

3x= 1.5

14×0.5+ 10× 1+2× 1.5

=7+ 10+3

=20

Answer: It takes 20 working hours to make 14 shirts, 10 pants and 2 tops.

10. Let AB be x kilometers apart.

x = 1/3(x/2+50)+400+ 1/2[ 1/3(x/2+50)+400-(x/2+50)]

6x = 2(x/2+50)+2400+3[ 1/3(x/2+50)+400-(x/2+50)]

6x = x+ 100+2400+(x/2+50+ 1200-3(x/2+50)

6x-x = 2500+x/2+ 1250-3x/2- 150

5x=3600-x

5x+x=3600

6x=3600

x=600

A: AB is 600 kilometers apart.