Explanation

Photorespiration required chloroplast, mitochondria and peroxisome to complete the process. At high temperature and high oxygen concentration, RuBPoxygenase oxidizes RuBP to produce phosphoglycolate then glyoxylate and then glycine. Two glycine molecule in addition to $\alpha$-ketoglutarate produce serine molecule. Therefore, to produce 20 serine molecules, 40 RuBP molecules are required.