We want the ratio of orange sea shells to pink sea shells to be 3: 7. we should use 40 shells. how many orange shells should we paint? ?

You should paint 12 sea shells.

The ratio 3:7 shows a precentage,30%:70%. the precent you want to paint orange is 30%. To change percentage into a value you move the decimal over two places to the left, 30.0% is 0.30. so you multiply 40 shells by 0.30 to get the answer of 12 shells.