\[ 3\times 5=15 \] donc \[ A=4+15=19 \]

\[ \boxed{A=19} \]

\[ 4+3=7 \] puis \[ B=7\times 5=35 \]

\[ \boxed{B=35} \]

\[ \frac{1}{4}+\frac{3}{4}=\frac{4}{4}=1 \]

\[ \boxed{1} \]

\[ \frac{2}{3}\times\frac{9}{4}=\frac{18}{12}=\frac{3}{2} \]

\[ \boxed{\frac{3}{2}} \]

\[ 20\%=0{,}20 \] donc \[ 150\times 0{,}20=30 \]

\[ \boxed{30} \]

\[ 1+0{,}10=1{,}10 \]

\[ 60\times1{,}10=66 \]

\[ \boxed{66\text{ €}} \]

\[ 1-0{,}15=0{,}85 \]

\[ 80\times0{,}85=68 \]

\[ \boxed{68\text{ €}} \]

\[ x=19-7=12 \]

\[ \boxed{x=12} \]

\[ x=\frac{45}{5}=9 \]

\[ \boxed{x=9} \]

\[ 2x=11-3=8 \] donc \[ x=4 \]

\[ \boxed{x=4} \]

\[ 398\times 21 \approx 400\times 20=8\,000 \]

Un ordre de grandeur est donc : \[ \boxed{8\,000} \]

(a) \[ 250\times0{,}12=30 \] Donc \(12\%\) de 250 vaut 30.

(b) \[ 250+30=280 \] ou directement \[ 250\times1{,}12=280 \]

\[ \boxed{280\text{ €}} \]