sub log2p5{ log( $_[0] ) / log( 2.5 ) };;

$X = log2p5( 10 ); 
$Y = log2p5( 15 ); 
$Z = log2p5( 10 + 15 ); 
print $X, $Y, $Z;;
2.51294159473206 2.95544864408182 3.51294159473206

print 2.51294159473206 + log2p5( 1 + 2.5**( 2.95544864408182 - 2.51294159473206 ) );;
3.51294159473206