Final answer:
The distance across the river is 200 feet.
Step-by-step explanation:
To find the distance across the river, we can set up a proportion using the given ratio. The ratio of d to 90 is the same as the ratio of 100 feet to 50 feet. So, we have:
d/90 = 100/50
Cross-multiplying, we can solve for d:
d = (100/50) * 90
Simplifying the expression, we get:
d = 200 feet