algebra, pre-algebra, diameter

A pipe outside diameter is the inside diameter plus the thickness of the pipe.

Just don't forget there are two wall on a pipe to be taken into account.

|----------------- 1 3/4" ----------------- |

|---|------------ 1 1/4 " ------------|---|

T T

O.D. = I.D. + 2 x T

Hope you can do it from there.