x = number of bags needed
The equation that will be set up is
10,000/20 = 140,000/x
cross multiplying gives
10,000x = 20*140,000
10,000x = 2,800,000
now divide both sides by 10,000 to fully isolate x
10,000x = 2,800,000
10,000x/10,000 = 2,800,000/10,000
x = 280
So you'll need 280 bags