Andrew has a collection of soda bottles. some of them are 12 ounce bottles, and others are 16 ounce bottles. if the collection contains 20 bottles, which hold a combined 300 ounces, how many of the soda bottles are 12-ounce bottles?

Use a for the 12 ounce bottles and b for the 16 ounce bottles. Now make two equations, one to show total ounces and one total bottles.
12a + 16b = 300 (12 ounces in a number of bottles plus 16 ounces in b number of bottles it 300 total bottles).
a + b = 20 (total number of bottles.)
Now use the substitution method to solve for a, the number of 12 ounce bottles:
a + b = 20
- a       - a
b = 20 - a (substitute 20 - a into the other equation for b)
12a + 16(20 - a) = 300  (Use distributive method
12a + 320 - 16a = 300  (Combine a's and subtrace 320)
       - 320          - 320
-4a = -20   (Divide by -4)
a = 5
There are 5 12 ounce bottles.

Speed required to cover second half of distance = 30 km/hr

Step-by-step explanation:

Total distance Steve traveled = 40 km

He covers the first half of the distance averaging a speed of 15 km/hr.

So he covers 20 km at a speed of 15 km/hr.

We need to find in order to average 20 km/hr for the entire trip , how many kilometers per hour must his average speed be during the second half of the trip.

So he need to cover second half distance in (2 - 1.33) hours = 0.67 hours.

Speed required to cover second half of distance = 30 km/hr