First, notice that if there are 4.2*10^7 hearbeats per year per person and there are 7,600,000,000 people in the world, then the total number of heartbeats in the world per year is equal to:

Rewrite the number 7,600,000,000 using scientific notation. Moving the decimal point 9 places to the left:

Therefore, the number of heartbeats per year can be expressed as:

Use the commutative property of multiplication to rewrite the product:

Multiply 4.2 times 7.6:

Use the properties of the exponents to rewrite (10^7)(10^9):

Move the decimal point one place to the left by increasing by 1 the exponent of the power of 10:

Comparing this number with the expression a x 10^b:

Therefore, there is a total of 3.192 x 10^17 human heartbeats per year.