diff --git a/docs/mcv4u7.md b/docs/mcv4u7.md
index a6b2809..8e2e25a 100644
--- a/docs/mcv4u7.md
+++ b/docs/mcv4u7.md
@@ -256,7 +256,7 @@ $$
 P(X=r)&=\text{binompdf}(n,p,r) \\
 P(X<r)&=\text{binomcdf}(n,p,r-1) \\
 P(X\leq r)&=\text{binomcdf}(n,p,r) \\
-P(a\leq X\leq b)&=\text{binomcdf}(n,p,b) - \text{binomcdf}(n,p,a)
+P(a\leq X\leq b)&=\text{binomcdf}(n,p,b) - \text{binomcdf}(n,p,a-1)
 \end{align*}
 $$