what number do you need to subtract 100 to make 38

All you need to do to figure out the number, kid, is to add 100 + 38

which equals 138

$${\mathtt{100}}{\mathtt{\,\small\textbf+\,}}{\mathtt{38}} = {\mathtt{138}}$$

Subrtract 138 minus 100, and you get 38

$${\mathtt{138}}{\mathtt{\,-\,}}{\mathtt{100}} = {\mathtt{38}}$$

so, in order to figure out the number when it comes to adding and subtracting, do the opposite (+ transforms to -, vice versa.) this also applies to multiplication and division. 

The number you need to match the equation is 138