Question 917000
There are two main temperature scales:

°F, the Fahrenheit Scale (used in the US), and 
°C, the Celsius Scale (part of the Metric System, used in most other countries)

They both measure the same thing (temperature!), but use different numbers:
 
Boiling water (at normal pressure) measures {{{100}}}° in Celsius, but {{{212}}}° in Fahrenheit

And as water freezes it measures {{{0}}}° in Celsius, but {{{32}}}° in Fahrenheit.


The scales start at a different number ({{{0}}} vs {{{32}}}), so we will need to add or subtract {{{32}}}. 

The scales rise at a different rate ({{{100}}} vs {{{180}}}), so we will also need to multiply.

And so, to convert:

from Celsius to Fahrenheit: first multiply by {{{180/100}}}, then add {{{32}}} 


from Fahrenheit to Celsius: first subtract {{{32}}}, then multiply by {{{100/180 }}}


But {{{180/100}}} can be simplified to {{{9/5}}}, 
and {{{100/180 }}}can be simplified to {{{5/9}}}, so this is the easiest way:


°C to °F multiply by {{{9}}}, then divide by{{{ 5}}}, then add {{{32}}}

°F to °C deduct {{{32}}}, then multiply by {{{5}}}, then divide by {{{9}}}


We can write each as a formula like this: 


Celsius to Fahrenheit: {{{(C * 9/5) + 32 = F}}}
Fahrenheit to Celsius: {{{(F -32) *5/9 = C}}}

so, you got
{{{(F -32) *5/9 = 30}}}

{{{(F -32) = 30*9/5}}}

{{{F -32 = 270/5}}}

{{{F -32 = 54}}}

{{{F  = 54+32}}}

{{{F  = 86}}}