Question

Stiind ca 2a+6b =90, calculati a+3b-18

Answer 1

$2a+6b=90 \\ \\ 2(a+3b)=90 \\ \\ a+3b= \frac{90}{2} \\ \\ a+3b=45 \\ \\ \boxed{a+3b-18=45-18=27}$

Answer 2

2a+6b=90  |:2   ⇒  a+3b=45
                                    ↓
                    a+3b -18 = 45 -18 = 27 .