For the heart rate :
n = number of heat beats = 36
t = time taken to measure = 30 s = 30/60 min = 0.5 min (since 1 min = 60 sec)
R = heart rate
Heart rate is given as

inserting the values

R = 72 min⁻¹
For uncertainty :
Δn = uncertainty in number of heat beats = ± 2
Δt = uncertainty in time taken = ± 0.4 s = ± 0.4/60 min = ± 0.0067 min (since 1 min = 60 sec)
ΔR = uncertainty in heart rate
uncertainty in Heart rate is given as

inserting the values

ΔR = ± 4.96 min⁻¹