（2÷3+3÷7+5÷21）÷21分之1÷0.28
=（2÷3+3÷7+5÷21）×21÷0.28
=（2÷3×21+3÷7×21+5÷21×21）÷0.28
=（14+9+5）÷0.28
=28÷0.28
=100