Let $s$ = number of sopranos and $a$ = number of altos.

$a = s + 37$ $s + a = 145$

Substitute:

$s + (s + 37) = 145$ $2s = 108$ $s = 54, \quad a = 91$

Ratio of altos to sopranos:

$a : s = 91 : 54$