Use a normal distribution calculator such as this to calculate this in a jiffy.

https://onlinestatbook.com/2/calculators/normal_dist.html

Enter the values of mean as 80, standard deviation as 9 and area as 0.4 (since 40% is 0.4) as shown below. The answer is 77.72 or approximately 77 heartbeats per min.

I found an answer from www.khanacademy.org

**Modeling data distributions** | **AP**®︎/College **Statistics** | Math | Khan ...

We'll measure the position of data within a distribution using percentiles and z-
scores, we'll ... and we'll look at one of the most important families of distributions
called **Normal** distributions. ... Unit: **Modeling data distributions**. **AP Stats**: UNC (
BI). ,. UNC‑1 (EU) ... **Normal calculations in reverse**Get 3 of 4 questions to level
up!

For more information, see **Modeling data distributions** | **AP**®︎/College **Statistics** | Math | Khan ...