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Example 4-5c Objective Compute mentally with percents

Lesson 4 Contents Example 1Use Fractions to Compute Mentally Example 2Use Fractions to Compute Mentally Example 3Use Decimals to Compute Mentally Example 4Use Decimals to Compute Mentally Example 5Use Proportion to Solve a Problem

Example 4-1a Compute 40% of 80 mentally. 4  8 Write the problem 40% of 80 Remember “of” means multiplication 00 Bring down the zeros Remember % means 100 so to take away the % sign divide by Answer: 32 1/5 Multiply the 4  8 32 Since there are 2 zeros in both numerator and denominator, cancel them out 32

Example 4-1b Compute 20% of 60 mentally. Answer: 12 1/5

Example 4-2a Compute of 75 mentally. 70% Write problem 66 % of Round to easy number 70 2/5 is about 70% 75 is about 70 or 80 (either will work) Remember: of means to multiply 

Example 4-2a Compute of 75 mentally. 70% 66 % of /5  Multiply the 7  7 7  7 49 Bring down the zeros 00 Remember % means 100 so to take away the % sign divide by Since there are 2 zeros in both numerator and denominator, cancel them out 49 Answer:

Example 4-2b Answer: 210 Compute of 300 mentally. 2/5

Example 4-3a Compute 10% of 65 mentally. 3/5 Write problem 10% of 65 Round to easy number 10% is already an easy number 10% Remember: of means to multiply 10%  65 is about 60 or 70 (either will work) 70

Example 4-3a Compute 10% of 65 mentally. 3/5 10% of 65 10% 10%  70 Multiply the 1  7 1  7 Bring down the zeros 700 Remember % means 100 so to take away the % sign divide by Since there are 2 zeros in both numerator and denominator, cancel them out 7 Answer:

Example 4-3b Answer: 1 Compute 10% of 13 mentally. 3/5

Example 4-4a Compute 1% of 304 mentally. 4/5 Write problem 1% of 304 Round to easy number 1% is already an easy number 1% 1%  Remember: of means to multiply 304 is about

Example 4-4a Answer: Compute 1% of 304 mentally. 4/5 1% of 304 1% 1%  300 Multiply the 1  3 1  3 3 Bring down the zeros 00 Remember % means 100 so to take away the % sign divide by Since there are 2 zeros in both numerator and denominator, cancel them out 3

Example 4-4b Answer: 2.5 Compute 1% of 244 mentally. 4/5

Example 4-5a TECHNOLOGY A company produces 2,500 of a particular printer. They later discover that 25% of the printers have defects. How many printers from this group have defects? Remember: Part = %  Whole 5/5 Define the variable: How many printers are defective Since all the printers are not defective, need to find the part x How many printers from this group have defects Convert 25% to a decimal x = 0.25

Example 4-5a TECHNOLOGY A company produces 2,500 of a particular printer. They later discover that 25% of the printers have defects. How many printers from this group have defects? Company produces 2,500 which is the total number of printers 5/5 x How many printers from this group have defects x = 0.25 x = 0.25  2,500 Since the variable is by itself, multiply 0.25  2,500 x = 625 Add dimensional analysis x = 625 defective printers Answer: 625 printers had defects

Example 4-5c TECHNOLOGY A company produces 1,400 of a particular monitor. They later discover that 20% of the monitors have defects. How many monitors from this group have defects? Answer: 280 defective monitors * 5/5