I can't understand the question of number 6 can you please make me understand - Maths - Fundamental Concepts

Question

I can't understand the question of number 6 can you please make me
understand

Josepaul Paulachan · Accepted Answer

Dear Studentwater in tank in morning = x water drained in afternoon = 31% of x water in evening = 138LThereforex-31x100=138(100-31)x100=13869x=13800x=200Therefore water in tank in morning = x=200LRegards