Calculating the change in relative humidity

[pondwire]

Homework Statement

the air temperature is -15 degrees Celsius and the relative humidity is 70 per cent. If the air is brought indoors and heated to 20 degrees Celsius with no added moisture, what is the new relative humidity?

Homework Equations

RH=e/es x 100% or ws=epsilon*es/p

The Attempt at a Solution

could not do it mathematically -- looked up on a chart and found that sat. vp is about 1.9 mb at 70 per cent RH, so multiplied 1.9 x .70 and got 13.3 mb. Then, divided that by 23.4 mb, from the 20 degree value, and got 57 per cent RH. Book says it's 6 per cent. Any ideas?

 

[rock.freak667]

Which chart did you use? If you used the psychometric chart, then you can easily find the new relative humidity. No moisture added means that the humidity ratio is the same.