The normal annual precipitation at stations X, A, B and C are 700 mm, 1000 mm, 900 mm and 800 mm respectively. If the storm precipitation at three station A, B and C were 100 mm, 90 mm and 80 mm respectively, then the storm precipitation for station X will be

Storm precipitation at three point
A B & C are 100mm , 90mm , 80mm respectively
Normal annual precipitation are
X, A, B , C are 700mm , 1000 mm, 900mm 800mm respectively.
Now we find out X Storm precipitation =
1/3* { 700*100/1000 + 700*90/900 + 700*80/800}
=( 70 + 70 + 70 ) /3
= 70 mm answer

1/3[700/1100×110+700/900×90+700/700×70] = 70 mm