Question 911557
In 3 packs of light bulbs, with each pack containing 12 light bulbs, Joaquín had
{{{3*12=36}}} light bulbs.
If Joaquín used as many bulbs as he needed to change all the bulbs in his house, and he has 15 bulbs left,
then he used {{{36-15=highlight(21)}}} bulbs.
If an equation was needed, we could say
{{{x}}}= number of bulbs Joaquín used. So,
{{{15=36-x}}} ---> {{{15+x=36}}} ---> {{{x=36-15}}} ---> {{{highlight(x=21)}}} .