61: What is the area of a circle 32-1/2 inches in diameter? Answer 830 sq. inches 62: What is the area of a circle 24 inches in diameter? Answer 452 sq. inches 63: What is the circumference of a circle whose diameter is 95 feet? Answer 298 ft. 64: What is the circumference of a circle whose diameter is 3.65 inches? Answer 11.5 inches 5. Ratio and Proportion. Example 65: 3 : 7 : : 4 : X or (3/7) = (4/x) Find X Set 3 on C scale over 7 on D scale. Read X on D scale under 4 on C scale. In fact, any number on the C scale is to the number directly under it on the D scale as 3 is to 7. PRACTICAL PROBLEMS SOLVED BY SLIDE RULE 1. Discount. A firm buys a typewriter with a list price of $150, subject to a discount of 20% and 10%. How much does it pay? A discount of 20% means 0.8 of the list price, and 10% more means 0.8 X 0.9 X 150 = 108. To do this on the slide rule, put the index of the C scale opposite 8 on the D scale and move the indicator to 9 on the C scale. Then move the slider till the right-hand index of the C scale is under the hairline. Now, move the indicator to 150 on the C scale and read the answer $108 on the D scale. Notice that in this, as in many practical problems, there is no question about where the decimal point should go. 2. Sales Tax. A man buys an article worth $12 and he must pay a sales tax of 1.5%. How much does he pay? A tax of 1.5% means he must pay 1.015 * 12.00. Set index of C scale at 1.015 on D scale. Move indicator to 12 on C scale and read the answer $12.18 on the D scale. A longer but more accurate way is to multiply 12 * .015 and add the result to $12. 3. Unit Price. A motorist buys 17 gallons of gas at 19.5 cents per gallon. How much does he pay? Set index of C scale at 17 on D scale and move indicator to 19.5 on C scale and read the answer $3.32 on the D scale. 4. Gasoline Mileage. An automobile goes 175 miles on 12 gallons of gas. What is the average gasoline consumption? Set indicator over 175 on D scale and move slider till 12 is under hair-line. Read the answer 14.6 miles per gallon on the D scale under the left-hand index of the C scale. 5. Average Speed. A motorist makes a trip of 256 miles in 7.5 hours. What is his average speed? Set indicator over 256 on D scale. Move slider till 7.5 on the C scale is under the hair-line. Read the answer 34.2 miles per hour under the right-hand index of the C scale. 6. Decimal Parts of an Inch. What is 5/16 of an inch expressed as decimal fraction? Set 16 on C scale over 5 on D scale and read the result .3125 inches on the D scale under the left-hand index of the C scale. 7. Physics. A certain quantity of gas occupies 1200 cubic centimeters at a temperature of 15 degrees C and 740 millimeters pressure. What volume does it occupy at 0 degrees C and 760 millimeters pressure? Volume = 1200 X (740/760) * (273/288) = 1100 cubic cm.