Explanation:
1. The ratio of cakes baked to hours worked is the slope of the line. Pick a point on the line, then divide the number of cakes by the number of hours. For example, (2 hours, 60 cakes) is a point on the line. So the ratio is:
60 cakes / 2 hours = 30 cakes : 1 hour
2. The airplane travels 780 miles in 4 hours. So the ratio is:
780 miles / 4 hours = 195 miles : 1 hour
So for each hour of travel, the plane travels 195 miles.
At 2 hours, the plane travels 2 × 195 = 390 miles.
At 8 hours, the plane travels 8 × 195 = 1560 miles.
At 12 hours, the plane travels 12 × 195 = 2340 miles.
It is a direct variation because the ratio of miles to hours is always the same.