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<html> Start Dosage (per day):<br> <input type="number" oninput="compute()" id="in_tabs" min=1 value=1><br> <select id="in_weeks" value=1 oninput="compute()"> <option value="1">Every week</option> <option value="2">Every 2 weeks</option> <option value="3">Every 3 weeks</option> <option value="4">Every 4 weeks</option> <option value="5">Every 5 weeks</option> </select> reduce dosage by <select id="in_reduce" value=1 oninput="compute()"> <option value="1">1 tablet</option> <option value="2">2 tablets</option> <option value="3">3 tablets</option> <option value="4">4 tablets</option> <option value="5">5 tablets</option> </select> <p id="result">7 tablets, over 1 week treatment duration</p> <script> //------------- !Javascript section! ------------------ function lazyMathsMethod(num_tabs, n_weeks, reduce_by) { var result = 0; var num_weeks = 0; while(num_tabs > 0) { result += num_tabs; num_tabs -= reduce_by; num_weeks++; } // multiply by number of days in week result *= 7; // multiply by reduction time period result *= n_weeks; // done! return [result, num_weeks]; } function compute() { // get the control with the number of tablets value in var in_tabs_control = document.getElementById('in_tabs'); var in_tabs = Number(in_tabs_control.value); // get the control with the number to reduce by each 'N' weeks var in_reduce_control = document.getElementById('in_reduce'); var in_reduce = Number(in_reduce_control.value); // The time period for the dosage reduction var in_weeks_control = document.getElementById('in_weeks'); var in_weeks = Number(in_weeks_control.value); if(in_reduce && in_reduce > 0) { // Do the maths! var result = lazyMathsMethod(in_tabs, in_weeks, in_reduce); // grab the place to write the result. var result_text = document.getElementById('result'); // write the result to the HTML element. result_text.innerHTML = result[0] + " tablets, over " + result[1] + " weeks treatment duration"; } } </script> </html>