Ideally these would be measurements with units.
#(a) 15.1+75.32=90.4# (When measurements are added or subtracted, the answer is rounded to the same number of decimal places as the number with the fewest decimal places used to get the result, in this case, 15.1, which has only one decimal place.)
#(b) (5.50xx10^8)(4xx10^5)=2xx10^14# (When multiplying or dividing measurements, the result is rounded to the least number of significant figures used to get the result. In this case #4xx10^5# has one significant figure, so the answer has one significant figure.)
#(c) 5.672xx10^(-6)+7.785xx10^(-6)=1.346xx10^(-5)# (When adding or subtracting numbers in scientific notation, the result is rounded so that it has the same number of decimal places as the least number of decimal places, in this case 3 for both numbers. Only the coefficient counts, not the exponent.)
#(d) 45.32xx2.3=1.0xx10^2# (Same as #(b)#: In this case, the result is rounded to two significant figures. Since it is 100, the only way to indicate that it has two significant figures is to write it in scientific notation.)