[SoonerATC]

Just keep it simple. Remember, if a plane is travelling at XYZ kts, that means it will travel XYZ nautical miles in one hour. Try to use equivalent fractions as much as possible.

Example: If N12345 is flying 450 kts, how far will it have travelled in 20 minutes?
Answer: 20 minutes is 1/3 of an hour, so find 1/3 of 450. In 20 minutes, N12345 would fly 150 nautical miles.

Example: If N12345 starts at 10,000 feet and is climbing at 500 feet per minute flying at 350 kts, what is its altitude after 70 miles?
Answer: 70 miles is 1/5 of 350. 1/5 of an hour is 12 minutes. 12 minutes x 500 feet per minute = 6000 feet. If it started at 10,000 feet, it's altitude after 70 miles is 16,000 feet.

Example: If N12345 is travelling west at 300 kts and N6789 is travelling east at 150 kts, how far apart will they be in 2.5 hours?
Answer: N12345 is flying 300 kts, so in 2.5 hours it would have travelled 750 miles. N6789 is flying 150 kts, so in 2.5 hours it would have travelled 375 miles. That means in 2.5 hours, they would be 1125 miles apart.

Use whatever system you need to, but keep it as simple and as logical as possible.