I suppose she couldn't have been too bright.

Rick

she could multiply 3-digit numbers in her head, and not take more than a second
or two. I saw her do, many times, such as calculate the price per pound of
5-3/4 ounces at $0.37 as compared to 8-3/8 ounces at $0.51 compared to 1 pound
2-1/2 ounces at $1.17 and pick up two of the 8-3/8 ounce size.

btw, she worked as a code breaker in WW2.