<span><span>2<span>H2O</span></span> → <span>2 <span>H2</span></span>+<span>O2
</span></span><span><span>5<span>O2</span></span>+<span>4P</span> → <span>2<span>P2O5 </span></span></span>
<span><span>P2O5</span>+<span>3<span>H2O</span></span> → <span>2<span>H3PO<span>4
</span></span></span></span>
40,5 -----х
65,37 ------ 98
х= 60,7
(60,7/245)*100% = 24,782% или правильней 25%.Удачи!