20. (5 points)
Da Mao and Er Mao together weigh 23 kilograms more than San Mao, Er Mao and San Mao together weigh 27 kilograms more than Da Mao, San Mao and Da Mao together weigh 37 kilograms more than Er Mao. Da Mao, Er Mao, and San Mao together weigh $\qquad$ kilograms.
$87$