I HAVE followed the letters concerning using a fax modem with interest, but I have yet to commit myself to buying one. A friend of mine has tried one out, but with not as much success as I would have expected. I would appreciate you answering a few questions regarding the problems he has experienced. 1. We have timed the sending of the same document, one sheet of A4 printing, by a regular fax machine and also using Winfax Pro. Why did the fax machine take only 53 seconds to send the document as against one minute 20 seconds when sent by Winfax? 2. When he received his telephone bill for a fax sent overseas, the charge was for one minute, as against the time on the send log of 44 seconds. Why the discrepancy in times again? I have noticed that when using the fax modem with Winfax, the initialisation and closing procedures take much longer than when using Bitfax. Why is this? Based on what I've seen, using a fax modem instead of a dedicated fax machine would not be cost-effective. GORDON KIM SING Tsim Sha Tsui A regular fax machine will begin transmitting a document as soon as it gets through to another machine on the other end of a fax or telephone line. A computer, on the other hand, has to process an image or text-based document before it transmits it via a fax/modem as a fax, and the machine only begins to do this once you send final commands and hit the enter key, or click on the final ''OK'' command button if you are using Windows. The process is similar to that which occurs when you print a document from a word processing program. This processing work takes time. The more complex the document to be sent - that is if it has graphics in it instead of being text only - the longer the processing time. The processing time is longer still if you are using a Windows-based fax software such as Winfax instead of a simple DOS-based one like Bitfax, since the graphical qualities of Windows and its related applications also slow down a computer. Your regular fax machine only took 53 seconds to transmit a page of A4 text because it dialed and immediately transmitted the document as soon as you hit the ''Start'' button on the device. Winfax, on the other hand, had to go through the process described above before it actually got round to transmitting the document, thus taking one minute 20 seconds to complete the job. The difference in time between the initialisation procedures of Winfax and Bitfax is explained by the speed with which your computer works with Windows software (Winfax) and DOS programs (Bitfax), the latter being quicker because it is less graphically orientated. I think the answer to your second question might have something to do with the way Winfax measures the transmission time of a document. The actual processing time taken before transmission begins is not factored into the measurements for its log. Instead, only the time from when the first bit of data starts flowing out of the computer on to the fax line to when the line is disconnected is measured. This happens even though some processing work may occur between the time the software gets a clear connection signal and when it actually starts transmitting. The telephone company, on the other hand, does not care about how long it takes to simply ''send'' a document. It measures the total time the line was open and, therefore, in use. Thus the discrepancy between your fax log and your phone bill. I HAVE heard the phrase ''what if'' used with reference to manipulating spreadsheets, and given a certain degree of importance. I am new to spreadsheets and would like to know what this means. Could you explain? NAME AND ADDRESS SUPPLIED I can try. Spreadsheet programs such as Microsoft Excel, Lotus 1-2-3 and Borland's Quattropro manipulate a grid of related numbers. Change one number and you cause a recalculation that can effect the values of one or all of the numbers in the spreadsheet. How they change is based upon the model you create. A spreadsheet model, or template, is your definition of the mathematical relationship between the numbers in each cell. Today's sophisticated spreadsheets can visually display your data via graphs and charts. This makes values easier to analyse and trends more apparent. But, with all these new features, it is still the consensus that a spreadsheet's strongest feature is being able to change a number to see how it affects the values of other numbers. This is known as playing ''what if''. Playing ''what if'' is, however, a tedious process. You select the cell that contains the value, type in the new number and press the enter key. After seeing the effect, you repeat the process to play again. To change numbers in other cell locations, you first have to find the cell and then play. And because the cells you want to change are typically dispersed throughout the model, you wind up spending most of your time scrolling rather than playing.