Question 137165
Let x = hours that the bricklayer worked, y = hours that the electrician worked 
know: (1) x + y = 90 and (2) 12x + 16y = 1350
x + y = 90 => x = 90 - y
Replace x by 90 - y in equation (2), so 12(90 - y) + 16y = 1350
Solve this for y. then plug it back in equation (1) to get x.