
Shortcut Trick
⇒ Take A = 50 (middle class)
⇒ Mean = 55 ⇒ Σf(d) = N(Mean − A) = 64 × 5 = 320
⇒ Σf(d) = (4×−40) + (19×−20) + (x×0) + (5×20) + (y×40)
⇒ = −160 − 380 + 100 + 40y = −440 + 40y
⇒ −440 + 40y = 320 ⇒ 40y = 760 ⇒ y = 19
⇒ x + y = 36 ⇒ x = 17
⇒ 7x + 5y = 7×17 + 5×19 = 214
∴ Answer = 214

Alternate Method
Given:
Mean = 55
Class intervals: 0–20, 20–40, 40–60, 60–80, 80–100
Frequencies: 4, 19, x, 5, y; Total = 64
Formula Used:
Mean = Σ(f × mid value) ÷ Σf
Calculation:
⇒ Mid values = 10, 30, 50, 70, 90
⇒ Total frequency ⇒ 4 + 19 + x + 5 + y = 64 ⇒ x + y = 36
⇒ Σfx = 4×10 + 19×30 + x×50 + 5×70 + y×90
⇒ = 40 + 570 + 50x + 350 + 90y
⇒ = 960 + 50x + 90y
⇒ Mean = 55 ⇒ (960 + 50x + 90y)/64 = 55
⇒ 960 + 50x + 90y = 3520
⇒ 50x + 90y = 2560
⇒ Divide by 10 ⇒ 5x + 9y = 256
⇒ Solve with x + y = 36
⇒ x = 36 − y
⇒ 5(36 − y) + 9y = 256
⇒ 180 − 5y + 9y = 256
⇒ 4y = 76 ⇒ y = 19
⇒ x = 36 − 19 = 17
⇒ 7x + 5y = 7×17 + 5×19 = 214
∴ Final Answer = 214

Additional Information
Assumed Mean Method: Σf(d) = N(Mean − A)
Mid Value: (Lower + Upper)/2
Mean Formula: Σfx / Σf
Exam Tip: Always form x + y equation first