90°-30°15′20〃= 15°51′×2= 55°28′+42°32′-13°27′= 176°52÷3=

90°-30°15′20″
=89°59′60″-30°15′20″
=(89°-30°)+(59′-15′)+(60″-20″)
=59°+44′+40″
=59°44′40″
15°51′×2
=(15°+51′)×2
=15°×2+51′×2
=30°+102′
=30°+60′+42′
=30°+1°+42′
=31°+42′
=31°42′
55°28′+42°32′-13°27′
=(55°+42°-13°)+(28′+32′-27′)
=84°+33′
=84°33′
176°52′÷3
=(174°+2°+52′)÷3
=(174°+120′+52′)÷3
=(174°+172′)÷3
=(174°+171′+60″)÷3
=174°÷3+171′÷3+60″÷3
=58°+57′+20″
=58°57′20″