Fluency exercise

Write out all of the equations you have. Which are the simplest looking equations? If you have an equation involving only two letters, can you write down the possible values those letters could take, knowing that each letter is a different number between $1$ and $9$?
Can you spot any equations which have only one possible combination of numbers? For example, if $a+b+c+d=10$, where $a$, $b$, $c$ and $d$ are whole numbers between $1$ and $9$, the only possible numbers are $1+2+3+4=10$. Can you use this to narrow down the possible values for each letter?