Question 142797
Let x=amount of time spent by the bricklayer
And let y=amount of time spent by the electrician

Now we are told the following

x+y=90----------------------------eq1
and
12x+16y=1350 ($ understood)-----------------------eq2

Multiply eq1 by 12:
12x+12y=1080--------------------eq1a
Subtract eq1a from eq2:

4y=270  divide each side by 4
y=67.5 hours----------------------------amount of time spent by the electrician
Substitute y=67.5 into eq1:

x+67.5=90  add 67.5 to each side
x+67.5-67.5=90-67.5  collect like terms

x=22.5 hours-------------------------------time spent by the bricklayer

CK

67.5+22.5=90
90=90
and
67.5*16+22.5*12=1350
1080+270=1350
1350=1350

Hope this helps---ptaylor