You would break up the numbers using expanded form, so 247 will be 200+40+7, then 358 would be 300+50+8. You would then add the hundreds, then tens, finally the ones. So you will do 200+300, which is 500, then 40+50, which is 90, finally 7+8, which is 15+ Finally, you would then add all of them up. So 500+90+15=605.
Hope this helps!!