Newbie learning Heat Exchanger

Clemens Khoo
TL;DR Summary
Hi guys, I got a question here for this Heat Exchanger.
May I know why the answer is 24.7? How?
"ln" stand for?
How to calculate?
ΔTLMTDcf = 20 - 30
ln20/30 ←(What is "In"?)
= 24.7°C ←(Why the answer is 24.7? How?)

Homework Helper
Hello @Clemens Khoo,

In the first place: the answer is NOT 24.7 but 24.7 °C
(I don't even know the question -- you really should post the full problem statement ! -- , but I bet my career on that).

Then: you want to look up LMTD to find out what the exercise is about. If you don't know, then either you've been sleeping or not attended at all.

Ln is natural logarithm, which google. The notation ##\operatorname{ln}## is for dimwits, excel users and engineers who would otherwise interpret the corrrect notation ##\log## as a logarithm base 10.
(*)

And ##\log(20/30) ## is not 24.7 !

Ah, I see: you meant to write $${20-30\over\log(20/30)}$$ and didn't notice PF posts ignore spaces​

for this Heat Exchanger.