We can round off the numbers to make the calculations easy.
1500+7500
=1000+500+7000+500
=1000+7000+500+500
=8000+1000
=9000
6300+4700
=6000+300+4000+700
=6000+4000+300+700
=10000+1000
=11000
4400+8600
=4000+400+8000+600
=4000+8000+400+600
=12000+1000
=13000