v-modle中绑定数据 input输入框后watch监控输入数据，自动计算差值

Question

<el-form-item label="和值：" prop="a">

<el-input v-model="formData.a" placeholder="请填写"></el-input>

</el-form-item>
<el-form-item label="值1：" prop="b">

<el-input v-model="formData.b" placeholder=""></el-input>

</el-form-item>
<el-form-item label="值2：" prop="c">

<el-input v-model="formData.c"  placeholder=""></el-input>

</el-form-item>

在watch中加：
'formData.b'(val) {

    if (this.formData.a && this.formData.b) {
      this.formData.c = parseFloat(this.formData.a) - parseFloat(this.formData.b);
    }
  },
  'formData.c'(val) {
    if (this.formData.a && this.formData.c) {
      this.formData.b = parseFloat(this.formData.a) - parseFloat(this.formData.c);
    }
  },

运行时输入和值6，b的值0.4后，b的值变为0.4000000000000000，如下图，怎么解决啊啊啊？