To factor the expression \( 40b + 50 \), we first look for the greatest common factor (GCF) of the two terms.
The coefficients are 40 and 50. The GCF of 40 and 50 is 10.
Now, we can factor out 10 from the expression:
\[ 40b + 50 = 10(4b + 5) \]
The factored form of the expression is:
\[ 10(4b + 5) \]
Thus, the answer is \( 10(4b + 5) \).