Answer:
a = 3
Step-by-step explanation:
mean is calculated as
mean = [tex]\frac{sum}{count}[/tex]
Here mean = 15, sum = 342 + a and count = 20 + a , then
[tex]\frac{342+a}{20+a}[/tex] = 15 ( multiply both sides by 20 + a )
342 + a = 15(20 + a) ← distribute
342 + a = 300 + 15a ( subtract a from both sides )
342 = 300 + 14a ( subtract 300 from both sides )
42 = 14a ( divide both sides by 14 )
3 = a
