1800 + 1500 + 1200 +1500
= (1800 + 1200 ) + ( 1500 + 1500 )
= 2000 + 2000
= 4000
1800+1500+1200+1500
=(1800+1200)+(1500+1500)
=3000+3000
=6000
1800 + 1500 + 1200 +1500
=(1500x2 ) + 1(1800+1200)
=3000+3000
=6000
1800+1500+1200+1500
=(1800+1200)+(1500+1500)
=2000+2000
=4000
1800+1500+1200+1500
=(1800+1200)+(1500+1500)
=2000+2000
=4000
1800+1500+1200+1500
=<1800+1200>+<1500+1500>
=2000+2000
=4000