\[\boxed{\mathbf{Стр.\ 52.\ }еуроки - ответы\ на\ пятёрку}\]
\[\boxed{\mathbf{14.}}\]
\[1)\ 520 + 470 =\]
\[= (520 + 400) + 70 =\]
\[= 920 + 70 = 990\]
\[520 + 470 =\]
\[= (500 + 400) + (20 + 70) =\]
\[= 900 + 90 = 990\]
\[520 + 470 = 52\ дес. + 47\ дес. =\]
\[= 99\ дес. = 990\]
\[780 - 260 =\]
\[= (780 - 200) - 60 =\]
\[= 580 - 60 = 520\]
\[780 - 260 =\]
\[= (700 - 200) + (80 - 60) =\]
\[= 500 + 20 = 520\]
\[780 - 260 = 78\ дес.\ - 26\ дес. =\]
\[= 52\ дес. = 520\]
\[2)\ 430 + 350 = 780\]
\[520 + 160 = 680\]
\[460 - 210 = 250\]
\[810 + 140 = 950\]
\[690 - 350 = 340\]
\[970 - 730 = 240\]
\[\boxed{\mathbf{15.}}\]
\[1)\ 600 - 30 = 570\ (л) -\]
\[= бензина\ во\ второй\ цистерне.\]
\[2)\ (600 + 570) - 500 =\]
\[= (600 - 500) + 570 =\]
\[= 670\ (л) - бензина\ осталось.\]
\[Ответ:\ 670\ л.\]