8. Simple applications, percentage, rates. Many problems involving percentage and rates are easily solved by means of the slide rule. One per cent (1%) of a number N is N × 1/100; hence 5% of N is N × 5/100, and, in general,  p% of N is pN/100. Hence to find 83% of 1872                               to 1872 on D set right index of C,                               push hairline to 83 on C,                               at the hairline read 1554 on D. Since (83/100) × 1872 is approximately (80/100) × 2000 = 1600, the answer is 1554. To find the answer to the question "M is what per cent of N?" we must find 100 M ÷ N. Thus, to find the answer to the question "87 is what per cent of 184.7?" we must divide 87 × 100 = 8700 by 184.7. Hence                               push hairline to 87 on D,                               draw 1847 of C under the hairline,                               opposite index of C read 471 on D. The rough calculation (9000/200) = 45 shows that the decimal point should be placed after the 7. Hence the answer is 47.1%. For a body moving with a constant velocity, distance = rate times time. Hence if we write d for distance, r for rate, and t for time, we have                               d = rt, or r = d/t, or t = d/r. To find the distance traveled by a car going 33.7 miles per hour for 7.75 hours, write d = 33.7 × 7.75, and                               to 337 on D set right index of C,                               push hairline to 775 on C,                               at hairline read 261 on D. Since the answer is near to 8 × 30 = 240 miles, we have d = 261 miles. To find the average rate at which a driver must travel to cover 287 miles in 8.75 hours, write r = 287 ÷ 8.75, and                               push hairline to 287 on D,                               draw 875 of C under the hairline,                               opposite the index of C read 328 on D. Since the rate is near 280 ÷ 10 = 28, we have r = 32.8 miles per hour. EXERCISES 1. Find (a)   86.3 per cent of 1826.            (b)   75.2 per cent of 3.46.            (c)   18.3 per cent of 28.7.            (d)   0.95 per cent of 483. 2. What percent of            (a)   69 is 18?            (b)   132 is 85?            (c)   87.6 is 192.8?            (d)   1027 is 28? 3. Find the distance covered by a body moving            (a)   23.7 miles per hour for 7.55 hours.            (b)   68.3 miles per hour for 1.773 hours.            (c)   128.7 miles per hour for 16.65 hours. 4. At what rate must a body move to cover            (a)   100 yards in 10.85 seconds.            (b)   386 feet in 25.7 seconds.            (c)   93,000,000 miles in 8 minutes and 20 seconds. 5. Find the time required to move            (a)   100 yards at 9.87 yards per second.            (b)   3800 miles at 128.7 miles per hour.            (c)   25,000 miles at 77.5 miles per hour.