This is probably an old one but I never heard it before and it cheered me up no end on a monday morning, so here goes.

Professor of maths sends his wife a text.
"Dear wife, ur 54 yrs old. You no longer satisfy my needs so when you get this I will be in a motel with my 18 yr old assistant, I'm sorry I will be home late".

Wife sends reply.
"Dear husband, you're also 54 and by the time you get this I'll also be in a motel with 18yr old toyboy. You're a mathematician so you'll know that 18 goes in to 54 a lot more times than 54 goes in to 18, so dont wait up floppy knob!!!"