How to solve dis equation???

SIR I AM ASKING HOW H IS 72???

PROBLEM ON ROWAN TO HALSEY - EFFECT OF CHANGE

STD TIME FOR A JOB IS 90HRS.THE HRLY RATE OF GUARANTEED WAGES IS 50RS.BECAUSE OF SAVING IN TIME .WORKER  A gets an effective hrly rate of rs 60 under Rowan premium bonus system.For the same savingnin time , calculate the hrly rate of wages that worker B will get under Halsey premium bonus system,assuming 40% to worker...

H/90 X (90-H) X 50 = 10H

By transferring 90 to rhs

H X (90-H) X 50 = 900H

H X (90-H) X 50 = 900H (BY CANCELLING BOTH SIDES COMMON THINGS)

(90-H) X 5 = 90

90-H = 18 ( BY TRANSFERRING 5 TO RHS)

Hence H = 72 

Or, 50 H Power 2=4500h-900h

Hence,         H=72