二十一分之八×87.5%-八分之七×二十一分之五
=8/21x7/8-7/8x5/21
=7/8x(8/21-5/21)
=7/8x1/7
=1/8
x+15%x=4.6
1.15x=4.6
x=4.6÷1.15
x=4