24)
= (25 x 10^7) + (3.8 x 10^7)
= (25 + 3.8) x 10^7
= 28.8 x 10^7
= 2.88 x 10^8
25)
= (670 x 10^2) - (6.1 x 10^2)
= (670 - 6.1) x 10^2
= 663.9 x 10^2
= 6.639 x 10^4
26)
= (3.2 + 5.4) x 10^3
= 8.6 x 10^3
27)
=(15 x 10^26) - (1.4 x 10^26)
= (15 - 1.4) x 10^26
= 13.6 x 10^26
= 1.36 x 10^27
28)
(1.72 x 10^4) + (8 x 10^6)
= (1.72 x 10^4) + (800 x 10^4)
= 801.72 x 10^4
= 8.0172 x 10^6
I don't agree with his answer 9.72 x 10^10. When adding or subtracting in scientific notations, the exponents must be the same so you can use the distributive property, then add or subtract the new coefficients
