Question

How do I find out how many years it will take? 

The San Andreas fault is a dextral (right-lateral) strike-slip fault that runs between Los Angeles and San Francisco. This means that Los Angeles is slowly approaching San Francisco. Assuming the average slip rate of 2.1 cm/year remains constant, in about how many years will Los Angeles be directly west of San Francisco? Los Angeles is presently 519 km from San Francisco. (Hint: 1 cm = 0.00001 km.)

Answer 1

519 km....ok, lets convert it to centimeters

there are 1000 meters in 1 km, so in 519 km, there are 519 * 1000 meters then.

there are 100 centimeters in 1 meter, so in (519 * 1000) meters, there are then (519 * 1000) * 100 centimeters.

that's how many centimeters Los Angeles is from San Francisco.

now, the slip rate is 2.1 cm per year.

how many times does 2.1 go into (519 * 1000 * 100)?

well, is just (519 * 1000 * 100) / 2.1.

that many years.