A, B, C three numbers, A (2/3) is equal to B (4/7), B (2/3) is equal to C (4/7), C is 13 greater than A, then B how many.
A、B、C三个数，A的(2/3)等于B的(4/7)，B的(2/3)又等于C的(4/7)，C比A大13，则B是多少．
<gadget id="calculator">13 / 42</gadget>
<output>13/42 = around 0.309524</output>

<gadget id="calculator">13 / (13/42)</gadget>
<output>42</output>

<result>42</result>