7+21:(15-8)=

Let's solve the mathematical expression step by step. The expression is:

7 + 21 ÷ (15 - 8)

According to the order of operations (PEMDAS/BODMAS), which stands for Parentheses/Brackets, Exponents/Orders, Multiplication and Division (from left to right), and Addition and Subtraction (from left to right), we need to do the operation inside the parentheses first, followed by division and then addition.

First, calculate the value inside the parentheses:

15 - 8 = 7

Now the expression looks as follows:

7 + 21 ÷ 7

Next, we perform the division:

21 ÷ 7 = 3

Now the expression is:

7 + 3

Finally, perform the addition:

7 + 3 = 10

So the final answer is 10.