15 degrees celsius= ____degrees Fahrenheit the formula I have says to multiply 5/9 X (212-32) =5/9 X180 =100 the example is 100degrees= 212 degrees F. how do you multiply 5/9?

Tc = (5/9)*(Tf-32); Tc = temperature in degrees Celsius, Tf = temperature in degrees Fahrenheit.

Multiply by 5 and then divide by 9.
Your formula seems sketchy to me.
Think with me and we will derive the formula without looking it up.
0 thru 100 degrees celsius represents 32 thru 212 degrees fahrenheit. There is a difference of 100 degrees between 0 and 100 degrees celsius. There is a difference of 180 degrees between 32 and 212 degrees fahrenheit.
Therefore, to convert from celsius to fahrenheit, a span of 100 degrees (given celsius) needs to fit into a span of 180 degrees (wanting fahrenheit). To cause this to happen, the temperature in celsius needs to be multiplied by 180 (the span we want) and then divided by 100 (the span we have).
However, 0 celsius is not 0 fahrenheit, it is 32. Therefore, the answer we get after multiplying by 180 and dividing by 100 will be 32 degrees lower than the actual answer. Therefore, 32 needs to be added to the answer.
Producing the following formula:
degreesFahrenheit = degreesCelsius multipliedBy 180 dividedBy 100 plus 32. degreesFahrenheit = degreesCelsius * 180 / 100 + 32. degreesFahrenheit = degreesCelsius * 180/100 + 32. degreesFahrenheit = (degreesCelsius * 180/100) + 32.
Often times people will reduce the fraction (an unnecessary step which hides understanding): degreesFahrenheit = (degreesCelsius * 9/5) + 32.

Example 5/9 x 180 would be written 5/9 x 108/1, then multiply straight across

5 x 180 = 900, 9 x 1 = 9 for a fraction of 900/9, or 900 divided by 9, which equals 100.