3 boys to 4 girls. You know you have 12 boys/ ?girls. Do the boys first. 12 divides into 3 4 times, so you would multiply girls by the same amount.
3 x 4 = 12
4 x 4 = 16
So, you would have 12 boys/ 16 girls. Ratio 12 to 16 or 12 : 16
Hope this helps you. :)