Tìm x biết hai lăm phần trăm nhân x trừ bảy mươi nhân mười phần mười một

* Hướng dẫn giải

Đáp án D

Ta có :

$$\begin{gathered} 25\%.x-70\frac{10}{11}:\left(\frac{131313}{151515}+\frac{131313}{353535}+\frac{131313}{636363}+\frac{131313}{999999}\right)=-5 \\ \frac{1}{4}.x-70\frac{10}{11}:\left(\frac{131313:10101}{151515:10101}+\frac{131313}{353535}+\frac{131313:10101}{636363:10101}+\frac{131313:10101}{999999:10101}\right)=-5 \\ 25\%.x-70\frac{10}{11}:\left(\frac{13}{15}+\frac{13}{35}+\frac{13}{63}+\frac{13}{99}\right)=-5 \\ 25\%.x-70\frac{10}{11}:\left[13\left(\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}\right)\right]=-5 \\ 25\%.x-70\frac{10}{11}:\left[\frac{13}{2}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\right)\right]=-5 \\ 25\%.x-70\frac{10}{11}:\left[\frac{13}{2}\left(\frac{1}{3}-\frac{1}{11}\right)\right]=-5 \\ 25\%.x-70\frac{10}{11}:\left(\frac{13}{2}.\frac{8}{33}\right)=-5 \\ 25\%.x-70\frac{10}{11}:\frac{52}{33}=-5 \\ 25\%.x-70\frac{10}{11}.\frac{33}{52}=-5 \\ 25\%.x-45=-5 \\ 25\%.x=40 \\ x=40:\frac{25}{100} \\ x=160 \end{gathered}$$