How many plums balance one pear

On a balance, one pear together with three apples matches ten plums, and one apple together with six plums matches one pear. How many plums alone balance one pear?

$7$. Substituting $p=a+6m$ into $p+3a=10m$ gives $4a=4m$, so an apple weighs the same as a plum and $p=7m$.