This is attempting to make the size of units before and after to be the same so that easier to solve. As such since before red is 4 units, the give away 36 beads is divided into 4. That is, every unit of red, 9 beads is given away. So before red is 4 smaller units + (9x4). Hence 1 big unit of after red will be 4 smaller units. Following this pattern, the before blue is 5 big units within each is 9 beads. Hence 5 smaller units + (9x5). So these smaller units will be same number in the before and after event. So following the after red pattern, the after blue will be 4 smaller units x 3 = 12 smaller units. So working is made compared between the before and after blue model to form the equation. Hope you understand.