There are 90 tons of cement on the construction site, 3.5 tons are used every day, and it takes b days. Use the formula containing letters to express the remaining tons.
工地上有90吨水泥，每天用去3.5吨，用了b天，用含有字母的式子表示剩下的吨数是多少吨．
<gadget id="calculator">90 - 3.5</gadget>
<output>86.5</output>

<result>86.5</result>