20/100 ·b=a
⇒b=a/20/100
⇒b=a·100/20
⇒b=100a/20
⇒b=5a;
a. x/100 ·a=b
⇒x/100 ·a=5a
⇒x/100=5a/a
⇒x/100=5[a/a]
⇒x/100=5/1
⇒x/100=5
⇒x=5·100
⇒x=500[la suta reprezinta numarul b din numarul a];
b. x/100 ·[a+b]=a
⇒x/100 ·[a+5a]=a
⇒x/100 ·6a=a
⇒x/100=a/6a
⇒x/100=[a/a]/6
⇒x/100=1/6
⇒x=1/6·100
⇒x=100/6
⇒x=16,[6][la suta reprezinta numarul a din a+b];