Chapter 4 – Combined Operation on the C and D Scale
4.1 Alternate Division and Multiplication
For a problem such as , if we multiply 414 by 2.62 and then divide by 545, the following steps would be required.
Fig 4-1
Fig 4-2
Note: By using the procedure above, we would have to hold the answer once with the hair line and move the slide twice. This could be reduced to only one move of the slide, with no need to hold the answer with the hair line, if we were first to divide 414 by 545, and then multiply by 2.62.
This saving is because the answer from a division is marked on the D scale by the left or right index of the C scale, thus allowing us to then multiply without moving the slide.
Fig 4-3
Example 1: (Fig 4-3)
The hair line is only used for convenience of reading values, and the slide is positioned once for the two calculations. These savings, apart from anything else, will increase accuracy and speed.
Example 2:
(i.e. approx.
therefore the answer is 102.9
Note: The above method necessitated one move of the slide and one holding of the answer with the hair line. To multiply the three numbers in the numerator and then divide by the two numbers in the denominator would mean four moves of the slide, instead of one. Thus the three progressive answers would have to be held instead of only one, as in example 2. For the alternating division and multiplication, the only time extra moves may be required is when we run off the end of the scale for multiplication.
The procedure outlined in 2.4 and 3.3 covers all that is required for combined operations.
Example:
= ‘569’
(i.e. approx. )
therefore the answer is 5.69
Exercise 4(a)
Locate the decimal points for the following:
4.3 Miscellaneous Problems
Exercise 4(b)
Exercise 4(c)