840*25÷7÷3
=840÷(7×3)×25
=840÷21×25
=40×25
=1000
（0.5-3分之1）×4分之3÷0.25
=（0.5-3分之1）×4分之3×4
=（0.5-3分之1）×3
=0.5×3-3分之1×3
=1.5-1
=0.5